Medical fluid sensors and related systems and methods

ABSTRACT

This disclosure relates to medical fluid sensors and related systems and methods. In some aspects, a medical fluid cartridge includes a body including a first portion defining a first fluid passageway and a second portion defining a second fluid passageway in fluid communication with the first fluid passageway. The body defines a gap between the first portion of the body and the second portion of the body such that the first portion of the body can be disposed in a radio frequency device while the second portion of the body remains outside the radio frequency device.

TECHNICAL FIELD

This disclosure relates to medical fluid sensors and related systems andmethods.

BACKGROUND

During hemodialysis, impurities and toxins are removed from the blood ofa patient by drawing the blood out of the patient through a blood accesssite, typically via a catheter, and then passing the blood through anartificial kidney (often referred to as a “dialyzer”). The artificialkidney includes a semi-permeable membrane that separates a first conduitfrom a second conduit. Generally, a dialysis solution (often referred toas a “dialysate”) flows through the first conduit of the dialyzer whilethe patient's blood flows through the second conduit of the dialyzer,causing impurities and toxins to be transferred from the blood to thedialysate through the semi-permeable membrane. The impurities and toxinscan, for example, be removed from the blood by a diffusion process.After passing through the dialyzer, the purified blood is then returnedto the patient.

Maintaining a substantially constant concentration of sodium in thepatient's blood throughout the hemodialysis treatment can help to reduceor prevent discomfort experienced by the patient. Therefore, sodiumconcentrations in the patient's blood are often monitored duringhemodialysis treatment. One way to detect the sodium concentration in apatient's blood is to connect a conductivity sensor to a blood line ofthe hemodialysis system and to determine the sodium concentration of thepatient's blood flowing through that blood line based on theconductivity measured by the conductivity sensor. Sodium levels in thedialysate can then be adjusted to maintain the sodium concentration ofthe patient's blood within a desired range.

SUMMARY

In one aspect of the invention, a method includes using a dialysis fluidpump of a dialysis machine to deliver dialysis fluid to a first portionof a cartridge that is positioned within a magnetic field, excitingatoms in the dialysis fluid in the first portion of the cartridge byapplying radio frequency energy to the dialysis fluid in the firstportion of the cartridge, receiving radio frequency energy generated bythe excited atoms in the dialysis fluid in the first portion of thecartridge, and determining a concentration of a substance in thedialysis fluid based on the received radio frequency energy generated bythe excited atoms in the dialysis fluid in the first portion of thecartridge.

In another aspect of the invention, a dialysis system includes a magnetassembly that generates a magnetic field and defines a cavity configuredto receive a first portion of a cartridge, a dialysis fluid pump that isoperable to pump dialysis fluid to the first portion of the cartridgewhen the first portion of the cartridge is disposed in the cavity of themagnet assembly, and a radio frequency device configured to receive thefirst portion of the cartridge when the first portion of the cartridgeis disposed in the cavity of the magnet assembly. The radio frequencydevice is operable to receive radio frequency energy generated byexcited atoms in the dialysis fluid in the first portion of thecartridge when the first portion of the cartridge is disposed in thecavity of the magnet assembly and dialysis fluid has been pumped to thefirst portion of the cartridge.

In an additional aspect of the invention, a method includes using amedical fluid pump to deliver medical fluid to a first portion of acartridge that is positioned within a magnetic field, exciting atoms inthe medical fluid in the first portion of the cartridge by applyingradio frequency energy to the medical fluid in the first portion of thecartridge, receiving radio frequency energy generated by the excitedatoms in the medical fluid in the first portion of the cartridge, anddetermining a concentration of a substance in the medical fluid based onthe received radio frequency energy generated by the excited atoms inthe medical fluid in the first portion of the cartridge.

In a further aspect of the invention, a medical system includes a magnetassembly that generates a magnetic field and defines a cavity configuredto receive a first portion of a cartridge, a medical fluid pump that isoperable to pump medical fluid to the first portion of the cartridgewhen the first portion of the cartridge is disposed in the cavity of themagnet assembly, and a radio frequency device configured to receive thefirst portion of the cartridge when the first portion of the cartridgeis disposed in the cavity of the magnet assembly. The radio frequencydevice is operable to receive radio frequency energy generated byexcited atoms in the medical fluid in the first portion of the cartridgewhen the first portion of the cartridge is disposed in the cavity of themagnet assembly and medical fluid has been pumped to the first portionof the cartridge.

Implementations can include one or more of the following features.

In some implementations, the dialysis fluid is blood.

In certain implementations, the dialysis fluid is dialysate.

In some implementations, the dialysate is spent dialysate.

In certain implementations, the method further includes delivering freshdialysate to the first portion of the cartridge, exciting atoms in thefresh dialysate in the first portion of the cartridge by applying radiofrequency energy to the fresh dialysate in the first portion of thecartridge, receiving radio frequency energy generated by the excitedatoms in the fresh dialysate in the first portion of the cartridge, anddetermining a concentration of the substance in the fresh dialysatebased on the received radio frequency energy generated by the excitedatoms in the fresh dialysate in the first portion of the cartridge.

In some implementations, the method further includes determining aconcentration of the substance in blood of a dialysis patient based onthe determined concentrations of the spent dialysate and the freshdialysate.

In certain implementations, the method further includes adjusting aconcentration of the substance in the fresh dialysate to match thedetermined concentration of the substance in the blood.

In some implementations, the substance is sodium.

In certain implementations, the dialysis fluid pump is a blood pump.

In some implementations, the dialysis fluid pump is a dialysate pump.

In certain implementations, the dialysis fluid is delivered to the firstportion of the cartridge while dialysis treatment is being carried outby the dialysis machine.

In some implementations, the concentration of the substance in thedialysis fluid is determined while dialysis treatment is being carriedout by the dialysis machine.

In certain implementations, the dialysis fluid is blood, and the methodfurther includes adjusting a concentration of the substance in dialysatebased on the determined concentration of the substance in the blood.

In some implementations, the concentration of the substance in thedialysate is adjusted to match the determined concentration of thesubstance in the blood.

In certain implementations, the method further includes adjusting theconcentration of the substance in the dialysis fluid if the determinedconcentration of the substance in the dialysis fluid falls outside of adesired range.

In some implementations, adjusting the concentration of the substance inthe dialysis fluid includes adding the substance to the dialysis fluidor adding a diluent to the dialysis fluid.

In certain implementations, the substance is sodium and adding thesubstance to the dialysis fluid includes adding a sodium chloridesolution to the dialysis fluid.

In some implementations, the radio frequency energy generated by theexcited atoms in the dialysis fluid in the first portion of thecartridge is received by a radio frequency device surrounding the firstportion of the cartridge.

In certain implementations, applying the radio frequency energy to thedialysis fluid in the first portion of the cartridge includes activatingthe radio frequency device.

In some implementations, the radio frequency device is a radio frequencycoil.

In certain implementations, the radio frequency coil is operated in atransmit mode while applying the radio frequency energy to the dialysisfluid in the first portion of the cartridge, and the radio frequencycoil is operated in a receiving mode while receiving the radio frequencyenergy generated by the excited atoms in the dialysis fluid in the firstportion of the cartridge.

In some implementations, operating the radio frequency coil in thetransmit mode includes applying electrical energy to the radio frequencycoil and operating the radio frequency coil in the receive mode includesceasing the application of electrical energy to the radio frequencycoil.

In certain implementations, the magnetic field is generated by a magnetassembly defining a cavity in which the radio frequency device and thefirst portion of the cartridge are disposed.

In some implementations, the magnet assembly includes a pair of magnetsattached to a frame.

In certain implementations, the frame includes two U-shaped members thatcooperate to form the cavity.

In some implementations, the method further includes passing thedialysis fluid through a first meandering fluid passageway defined bythe cartridge prior to delivering the dialysis fluid to the firstportion of the cartridge. The first meandering fluid passageway ispositioned within the magnetic field.

In certain implementations, the first meandering fluid passageway is aU-shaped fluid passageway.

In some implementations, the dialysis fluid is in the first meanderingfluid passageway for a sufficient period of time to polarize nuclei ofthe atoms.

In certain implementations, the dialysis fluid is in the firstmeandering fluid passageway for at least 150 milliseconds (e.g., 150milliseconds to 300 milliseconds).

In some implementations, the first meandering fluid passageway ispositioned outside a radio frequency device that applies the radiofrequency energy to the dialysis fluid in the first portion of thecartridge.

In certain implementations, the first portion of the cartridge defines asecond meandering fluid passageway.

In some implementations, the second meandering fluid passageway is aU-shaped fluid passageway.

In certain implementations, the dialysis fluid is in the secondmeandering fluid passageway for a sufficient period of time for theatoms in the dialysis fluid to be excited by the applied radio frequencyenergy and for the radio frequency energy generated by the excited atomsto be received.

In some implementations, the dialysis fluid is in the second meanderingfluid passageway for at least 150 milliseconds (e.g., 150 millisecondsto 300 milliseconds).

In certain implementations, the dialysis fluid is delivered to the firstportion of the cartridge at a rate of 50 milliliters per minute to 200milliliters per minute.

In some implementations, the dialysis fluid flows through the firstportion of the cartridge.

In certain implementations, the dialysis fluid flows through the firstportion of the cartridge at a rate of 50 milliliters per minute to 200milliliters per minute.

In some implementations, the method further includes passing thedialysis fluid through a first meandering fluid passageway defined bythe cartridge before the dialysis fluid flows through the first portionof the cartridge. The first meandering fluid passageway is positionedwithin the magnetic field.

In certain implementations, the dialysis fluid flows through the firstmeandering fluid passageway at a first flow rate and passes through thefirst portion of the cartridge at a second flow rate that is less thanthe first flow rate.

In some implementations, the dialysis fluid flows through the firstmeandering fluid passageway at a rate of 50 milliliters per minute to200 milliliters per minute.

In certain implementations, the dialysis fluid resides substantiallystagnantly within the first portion of the cartridge for a period oftime.

In some implementations, the dialysis fluid is dialysate.

In certain implementations, a first portion of the dialysis fluid isdelivered to the first portion of the cartridge, and a second portion ofthe dialysis fluid passes through a second portion of the cartridge

In some implementations, the second portion is positioned outside aradio frequency device that applies the radio frequency energy to thedialysis fluid in the first portion, and the second portion is at leastpartially positioned outside a magnet assembly that generates themagnetic field.

In certain implementations, the second portion of the cartridge definesa fluid passageway that bypasses the first portion of the cartridge.

In some implementations, the fluid passageway that bypasses the firstportion of the cartridge is straight.

In certain implementations, the first portion of the dialysis fluidflows through the first portion of the cartridge at a slower rate thanthe second portion of the dialysis fluid passes through the secondportion of the cartridge.

In some implementations, the first portion of the dialysis fluid flowsthrough the first portion of the cartridge at a rate of 50 millilitersper minute to 200 milliliters per minute.

In certain implementations, the second portion of the dialysis fluidflows through the second portion of the cartridge at a rate of 400milliliters per minute to 600 milliliters per minute.

In some implementations, the concentration of the substance in thedialysis fluid is determined as a function of (i) the radio frequencyenergy generated by the excited atoms in the dialysis fluid in the firstportion of the cartridge and (ii) a volume of the first portion of thecartridge.

In certain implementations, the magnet assembly includes a pair ofmagnets attached to a frame.

In some implementations, the frame includes two U-shaped members thatcooperate to form the cavity.

In certain implementations, the radio frequency device is furtheroperable to apply radio frequency energy to dialysis fluid in the firstportion of the cartridge to excite the atoms in the dialysis fluid inthe first portion of the cartridge.

In some implementations, the radio frequency device is a radio frequencycoil.

In certain implementations, the dialysis system further includes acontroller in communication with the radio frequency device. Thecontroller is configured to determine a concentration of a substance inthe dialysis fluid based on the received radio frequency energygenerated by the excited atoms in the dialysis fluid in the firstportion of the cartridge.

In some implementations, the controller is configured to determine theconcentration of the substance in the dialysis fluid as a function of(i) the radio frequency energy generated by the excited atoms in thedialysis fluid in the first portion of the cartridge and (ii) a volumeof the first portion of the cartridge.

In certain implementations, the dialysis fluid pump is a blood pump.

In some implementations, the dialysis fluid pump is a dialysate pump.

In certain implementations, the dialysis system further includes thecartridge.

In some implementations, the cartridge defines a first meandering fluidpassageway in fluid communication with the first portion of thecartridge, and the first portion of the cartridge and the firstmeandering fluid passageway of the cartridge are configured to bedisposed within the cavity of the magnet assembly.

In certain implementations, the first meandering fluid passageway is aU-shaped fluid passageway.

In some implementations, the cartridge is configured such that the firstmeandering fluid passageway is disposed outside the radio frequencydevice when the first portion of the cartridge is disposed in the radiofrequency device.

In certain implementations, the first portion of the cartridge defines asecond meandering fluid passageway.

In some implementations, the second meandering fluid passageway is aU-shaped fluid passageway.

In certain implementations, the cartridge defines a fluid inlet port viawhich the dialysis fluid enters the cartridge. The fluid inlet port hasa first flow area and the first meandering fluid passageway has a secondflow area that is smaller than the first flow area.

In some implementations, the medical fluid is dialysis fluid.

In certain implementations, the medical fluid pump is a dialysis fluidpump of a dialysis machine.

In some implementations, the medical fluid pump is a dialysis fluid pumpof a dialysis machine.

In certain implementations, the magnet assembly includes a pair ofmagnets attached to a frame.

In some implementations, the frame includes two U-shaped members thatcooperate to form the cavity.

In one aspect of the invention, a method includes reading an indicia ofa medical fluid cartridge to determine a volume of a fluid passageway ofthe medical fluid cartridge indicated by the indicia, receiving radiofrequency energy generated by excited atoms in medical fluid in thefluid passageway of the medical fluid cartridge, and determining aconcentration of a substance in the medical fluid based on thedetermined volume of the fluid passageway of the medical fluid cartridgeindicated by the indicia and the received radio frequency energygenerated by the excited atoms in the medical fluid in the fluidpassageway of the medical fluid cartridge.

In another aspect of the invention, a method includes determining avolume of a fluid passageway of a medical fluid cartridge and applyingan indicia to the cartridge. The indicia is indicative of the determinedvolume of the cartridge, and the indicia is machine readable.

In an additional aspect of the invention, a method includes measuring aquantity of a first substance in a reference fluid in a reference fluidcartridge, measuring a quantity of a second substance in the referencefluid in the reference fluid cartridge, measuring a quantity of thefirst substance in a medical fluid in a medical fluid cartridge,measuring a quantity of the second substance in the medical fluid in themedical fluid cartridge, and determining a concentration of the secondsubstance in the medical fluid based on the measured quantities of thefirst and second substances in the reference fluid and the medicalfluid.

Implementations can include one or more of the following features.

In certain implementations, the method further includes determining anactual volume of the fluid passageway of the medical fluid cartridge andapplying the indicia to the medical fluid cartridge. The indicia isindicative of the determined actual volume of the fluid passageway ofthe medical fluid cartridge.

In some implementations, determining the actual volume of the fluidpassageway of the medical fluid cartridge includes measuring the actualvolume of the fluid passageway of the medical fluid cartridge.

In certain implementations, the actual volume of the fluid passageway ofthe medical fluid cartridge is measured using a contact probe.

In some implementations, the actual volume of the fluid passageway ofthe medical fluid cartridge is measured using a laser.

In certain implementations, the indicia of the medical fluid cartridgeis read by a machine.

In some implementations, the machine is a barcode reader and the indiciais a barcode.

In certain implementations, the radio frequency energy generated by theexcited atoms in the medical fluid in the fluid passageway of themedical fluid cartridge is received by a sensor assembly.

In some implementations, prior to determining the concentration of thesubstance in the medical fluid, the sensor assembly is used to determinea concentration of the substance in a reference fluid in a referencefluid cartridge and the sensor assembly is calibrated based on thedetermined concentration of the substance in the reference fluid in thereference fluid cartridge.

In certain implementations, calibrating the sensor assembly includescomparing the concentration of the substance in the reference fluid inthe reference fluid cartridge as determined by the sensor assembly to aknown concentration of the substance in the reference fluid in thereference fluid cartridge.

In some implementations, the sensor assembly includes a magnet assemblythat defines a cavity and is configured to generate a magnetic fieldwithin the cavity, and the sensor assembly includes a radio frequencydevice that is disposed in the cavity of the magnet assembly and isconfigured to receive the radio frequency energy generated by theexcited atoms in the medical fluid in the fluid passageway of themedical fluid cartridge.

In certain implementations, the method further includes exciting theatoms in the medical fluid in the fluid passageway of the medical fluidcartridge by applying radio frequency energy to the medical fluid in thefluid passageway of the medical fluid cartridge.

In some implementations, the radio frequency energy generated by theexcited atoms in the medical fluid in the fluid passageway of themedical fluid cartridge is received by a radio frequency devicesurrounding the fluid passageway of the medical fluid cartridge.

In certain implementations, the method further includes applying radiofrequency energy to the medical fluid in the fluid passageway of themedical fluid cartridge to excite the atoms in the medical fluid in thefluid passageway of the medical fluid cartridge. Applying the radiofrequency energy to the medical fluid in the fluid passageway of themedical fluid cartridge includes activating the radio frequency device.

In some implementations, the radio frequency device is a radio frequencycoil.

In certain implementations, the radio frequency coil is operated in atransmit mode while applying the radio frequency energy to the medicalfluid in the fluid passageway of the medical fluid cartridge, and theradio frequency coil is operated in a receiving mode while receiving theradio frequency energy generated by the excited atoms in the medicalfluid in the fluid passageway of the medical fluid cartridge.

In some implementations, operating the radio frequency coil in thetransmit mode includes applying electrical energy to the radio frequencycoil and operating the radio frequency coil in the receive mode includesceasing the application of electrical energy to the radio frequencycoil.

In certain implementations, the method further includes adjusting theconcentration of the substance in the medical fluid if the determinedconcentration of the substance in the medical fluid falls outside of adesired range.

In some implementations, the medical fluid is dialysis fluid.

In certain implementations, the dialysis fluid is blood.

In some implementations, the dialysis fluid is dialysate.

In certain implementations, the substance is sodium.

In some implementations, determining the volume of the fluid passagewayof the medical fluid cartridge includes measuring the volume of thefluid passageway of the medical fluid cartridge.

In certain implementations, the volume of the fluid passageway of themedical fluid cartridge is measured using a contact probe.

In some implementations, the volume of the fluid passageway of themedical fluid cartridge is measured using a laser.

In certain implementations, the indicia is a barcode.

In some implementations, the medical fluid cartridge is a dialysis fluidcartridge.

In certain implementations, the dialysis fluid cartridge is a bloodcartridge.

In some implementations, the dialysis fluid cartridge is a dialysatecartridge.

In certain implementations, concentrations of the first and secondsubstances in the reference fluid are known.

In some implementations, a concentration of the first substance in themedical fluid is known.

In certain implementations, measuring the quantities of the first andsecond substances in the reference fluid includes receiving radiofrequency energy generated by excited atoms in the reference fluid inthe reference fluid cartridge, and measuring the quantities of the firstand second substances in the medical fluid includes receiving radiofrequency energy generated by excited atoms in the medical fluid in themedical fluid cartridge.

In some implementations, the method further includes exciting the atomsin the reference fluid by applying radio frequency energy to thereference fluid in the reference fluid cartridge, and exciting the atomsin the medical fluid by applying radio frequency energy to the medicalfluid in the medical fluid cartridge.

In certain implementations, the radio frequency energy generated by theexcited atoms in the reference fluid in the reference fluid cartridge isreceived by a radio frequency device surrounding a portion of thereference fluid cartridge, and the radio frequency energy generated bythe excited atoms in the medical fluid in the medical fluid cartridge isreceived by a radio frequency device surrounding a portion of themedical fluid cartridge.

In some implementations, a single radio frequency device receives theradio frequency energy generated by the excited atoms in the referencefluid in the reference fluid cartridge and the radio frequency energygenerated by the excited atoms in the medical fluid in the medical fluidcartridge.

In certain implementations, the single radio frequency device is a radiofrequency coil.

In some implementations, the radio frequency coil is operated at a firstfrequency to measure the quantities of the first substance in thereference fluid and the medical fluid and is operated at a secondfrequency to measure the quantities of the second substance in thereference fluid and the medical fluid.

In certain implementations, the method further includes exciting theatoms in the reference fluid by applying radio frequency energy to thereference fluid in the reference fluid cartridge by activating the radiofrequency device that receives the radio frequency energy generated bythe excited atoms in the reference fluid in the reference fluidcartridge, and exciting the atoms in the medical fluid by applying radiofrequency energy to the medical fluid in the medical fluid cartridge byactivating the radio frequency device that receives the radio frequencyenergy generated by the excited atoms in the medical fluid in themedical fluid cartridge.

In some implementations, a single radio frequency device receives theradio frequency energy generated by the excited atoms in the referencefluid in the reference fluid cartridge and the medical fluid in themedical fluid cartridge and applies the radio frequency energy to thereference fluid in the reference fluid cartridge and the medical fluidin the medical fluid cartridge.

In certain implementations, the single radio frequency device is a radiofrequency coil.

In some implementations, the radio frequency coil is operated at a firstfrequency to measure the quantities of the first substance in thereference fluid and the medical fluid and is operated at a secondfrequency to measure the quantities of the second substance in thereference fluid and the medical fluid.

In certain implementations, the radio frequency energy generated by theexcited atoms in the reference fluid in the reference fluid cartridgeand the radio frequency energy generated by the excited atoms in themedical fluid in the medical fluid cartridge is received by a sensorassembly.

In some implementations, prior to determining the concentration of thesecond substance in the medical fluid, the sensor assembly is used todetermine a concentration of one of the first and second substances inthe reference fluid in the reference fluid cartridge and the sensorassembly is calibrated based on the determined concentration of the oneof the first and second substances in the reference fluid in thereference fluid cartridge.

In certain implementations, calibrating the sensor assembly includescomparing the concentration of the one of the first and secondsubstances in the reference fluid in the reference fluid cartridge asdetermined by the sensor assembly to a known concentration of the one ofthe first and second substances in the reference fluid in the referencefluid cartridge.

In some implementations, the sensor assembly includes a magnet assemblythat defines a cavity and is configured to generate a magnetic fieldwithin the cavity, and the sensor assembly includes a radio frequencydevice that is disposed in the cavity of the magnet assembly and isconfigured to receive the radio frequency energy generated by theexcited atoms in the medical fluid in the medical fluid cartridge.

In certain implementations, the radio frequency device is a dual tunedradio frequency coil.

In some implementations, the method further includes adjusting theconcentration of the second substance in the medical fluid if thedetermined concentration of the second substance in the medical fluidfalls outside of a desired range.

In certain implementations, the first substance is hydrogen and thesecond substance is sodium.

In some implementations, the medical fluid is dialysis fluid.

In certain implementations, the dialysis fluid is blood.

In some implementations, the dialysis fluid is dialysate.

In certain implementations, the reference fluid is a saline solutionhaving a known concentration of hydrogen and sodium.

In some implementations, the second substance of the medical fluid issodium.

In one aspect of the invention, a circuit includes a radio frequencycoil tuned to at least one frequency and at least one switching circuitdirectly connected to the radio frequency coil. The radio frequency coilis characterized by a high impedance.

In another aspect of the invention, a dialysis machine includes adialysis fluid pump, a radio frequency coil tuned to at least onefrequency, and at least one switching circuit directly connected to theradio frequency coil. The radio frequency coil is characterized by ahigh impedance.

Implementations can include one or more of the following features.

In certain implementations, the high impedance is an impedance ofgreater than 10K ohms.

In some implementations, the at least one switching circuit isolates afirst set of components for transmitting signals from a second set ofcomponents for receiving signals.

In certain implementations, the at least one switching circuit includesat least one high voltage transistor.

In some implementations, the at least one high voltage transistorincludes a transistor which maintains a switching state when a voltageof at least 100 volts is applied to an input.

In certain implementations, a low noise amplifier is directly connectedto the at least one switching circuit.

In some implementations, the circuit includes a low noise amplifiercharacterized by an impedance that is ten times the impedance of theradio frequency coil.

In certain implementations, the radio frequency coil is tuned to both afirst frequency and a second frequency, where the first frequency is afrequency of sodium molecules and the second frequency is a frequency ofhydrogen molecules.

In some implementations, the circuit includes a first set of componentsfor receiving signals at the first frequency and a second set ofcomponents for receiving signals at the second frequency.

In certain implementations, the first frequency is 6.5 to 11 megahertzand the second frequency is 25 to 42 megahertz.

In some implementations, the dialysis fluid pump is a blood pump.

In certain implementations, the dialysis machine is a hemodialysismachine.

In one aspect of the invention, a nuclear magnetic resonance deviceincludes a support frame, a first magnet connected to the support frame,a second magnet connected to the support frame in a manner such that thesecond magnet is disposed within the magnetic field of the first magnetand a magnetic attraction exists between the first magnet and the secondmagnet, and a spacer disposed between the first magnet and the secondmagnet. The spacer is configured to maintain a space between the firstmagnet and the second magnet. The spacer includes a first side thatfaces the first magnet and a second side that is opposed to the firstside and faces the second magnet. The spacer has a shape that orientsthe first magnet relative to the second magnet in a manner such that apole face of the first magnet is maintained substantially parallel to apole face of the second magnet.

In another aspect of the invention, a dialysis system includes adialysis fluid circuit and a device for measuring a concentration of asubstance in a sample of dialysate fluid taken from the dialysis fluidcircuit. The device includes a support frame, a first magnet connectedto the support frame, a second magnet connected to the support frame ina manner such that the second magnet is disposed within the magneticfield of the first magnet and a magnetic attraction exists between thefirst magnet and the second magnet, and a spacer disposed between thefirst magnet and the second magnet. The spacer includes a first sidethat contacts the first magnet and a second side that is opposed to thefirst side and contacts the second magnet. The first side and the secondside define therebetween an interior space configured to receive thesample. The device also includes a radio frequency coil supported on thespacer so as enclose a portion of the interior space. The radiofrequency coil is configured to transmit a radio frequency signal to andreceive a radio frequency signal from the sample. The spacer has aperipheral shape that orients the first magnet relative to the secondmagnet in a manner such that a pole face of the first magnet ismaintained substantially parallel to a pole face of the second magnet.

In a further aspect of the invention, a device for measuring aconcentration of a substance in a sample includes a magnet supportstructure including a first frame member and a second frame member, afirst magnet supported on the first frame member, and a second magnetsupported on the second frame member in such a way that a magneticattraction exists between the first magnet and the second magnet. Themagnet support structure supports the first magnet in a spaced apartrelationship relative to the second magnet such that the first framemember the second frame member cooperate to substantially surround boththe first magnet and the second magnet. A first air gap exists betweenthe first magnet and the second magnet, a second air gap exists betweenthe first frame member and the second frame member, and a third air gapexists between the first frame member and the second frame member at alocation spaced apart from the first air gap and the second air gap. 19

In an additional aspect of the invention, a device for measuring aconcentration of a substance in a sample includes a first frame portionhaving a U-shape including a first frame base, a first frame armextending from one end of the first frame base in a directionperpendicular to the first frame base, and a second frame arm extendingfrom another end of the first frame base in a direction perpendicular tothe first frame base. The device also includes a second frame portionhaving a U-shape including a second frame base, a third frame armextending from one end of the second frame base in a directionperpendicular to the second frame base, and a fourth frame arm extendingfrom another end of the second frame base in a direction perpendicularto the second frame base. A first magnet is connected to the first framebase and resides between the first frame arm and the second frame arm. Asecond magnet is connected to the second frame base and resides betweenthe third frame arm and the fourth frame arm. The second magnet disposedwithin the magnetic field of the first magnet in such a way that amagnetic attraction exists between the first magnet and the secondmagnet. The first frame portion is arranged relative to the second frameportion in a manner such that a free end of the first frame arm faces afree end of the third frame arm and is spaced apart from the third framearm, and a free end of the second frame arm faces a free end of thefourth frame arm and is spaced apart from the fourth frame arm.

In yet another aspect of the invention, a dialysis system includes adialysis fluid circuit, and a device for measuring a concentration of asubstance in a sample. The device includes a magnet support structureincluding a first frame member and a second frame member. A first magnetis supported on the first frame member, and a second magnet is supportedon the second frame member in such a way that a magnetic attractionexists between the first magnet and the second magnet. The magnetsupport structure supports the first magnet in a spaced apartrelationship relative to the second magnet such that the first framemember and second frame member cooperate to substantially surround boththe first magnet and the second magnet. A first air gap exists betweenthe first magnet and the second magnet, a second air gap exists betweenthe first frame member and the second frame member, and a third air gapexists between the first frame member and the second frame member at alocation spaced apart from the first air gap and the second air gap.

In another aspect of the invention, a nuclear magnetic resonance deviceincludes a first magnet, a second magnet disposed adjacent to the firstmagnet in such a way that a first space exists between the first magnetand the second magnet and an attractive magnetic field exists in thespace, and a radio frequency coil assembly disposed in the space. Theradio frequency coil assembly is configured to transmit a radiofrequency signal to, and receive a radio frequency signal from, a sampledisposed in the space. A first non-magnetic, electrically-conductivemember is disposed between the radio frequency coil assembly and thefirst magnet, and a second non-magnetic, electrically-conductive memberis disposed between the radio frequency coil assembly and the secondmagnet.

In a further aspect of the invention, a device for measuring aconcentration of a substance in a sample includes a first magnet, asecond magnet disposed within the magnetic field of the first magnet insuch a way that a magnetic attraction exists between the first magnetand the second magnet, and a spacer disposed between the first magnetand the second magnet. The spacer is configured to maintain a spacebetween the first magnet and the second magnet. The spacer includes aspacer first side that faces the first magnet and a spacer second sidethat is opposed to the spacer first side and faces the second magnet.The spacer first side and the spacer second side define therebetween arecess that is configured to receive the sample. A radio frequency coilis supported by the spacer so as to surround at least a portion of therecess. The radio frequency coil is configured to transmit a radiofrequency signal to, and receive a radio frequency signal from, thesample. A first non-magnetic, electrically-conductive plate is disposedbetween the spacer first side and the first magnet, and a secondnon-magnetic, electrically-conductive plate is disposed between thespacer second side and the second magnet.

In another aspect of the invention, a dialysis system includes adialysis fluid circuit and a device for measuring a concentration of asubstance in a sample. The device includes a first magnet, a secondmagnet disposed within the magnetic field of the first magnet in such away that a magnetic attraction exists between the first magnet and thesecond magnet, and a spacer disposed between the first magnet and thesecond magnet. The spacer is configured to maintain a space between thefirst magnet and the second magnet, and the spacer defines a recessconfigured to receive the sample. A radio frequency coil is supported bythe spacer within the magnetic field and is configured to transmit aradio frequency signal to, and receive a radio frequency signal from,the sample. A first non-magnetic, electrically-conductive member isdisposed between the radio frequency coil and the first magnet, and asecond non-magnetic, electrically-conductive member is disposed betweenthe radio frequency coil and the second magnet.

In some implementations, the first side contacts the first magnet, andthe second side contacts the second magnet.

In certain implementations, the first side and the second side definetherebetween an interior space configured to receive a test sample.

In some implementations, the first side of the spacer and the secondside of the spacer are planar, and a plane defined by the first side ofthe spacer is substantially parallel to a plane defined by the secondside of the spacer.

In certain implementations, the pole face of the first magnet is angledrelative to the pole face of the second magnet by no more than 0.2degrees

In some implementations, the spacer is formed of a ceramic material.

In certain implementations, the spacer includes a pair of non-magnetic,electrically-conductive plates, and the spacer is sandwiched between thepair of plates such that a first surface of a first plate of the pair ofplates contacts the first magnet, and a surface opposed to the firstsurface of the first plate contacts a first side of the spacer, and afirst surface of a second plate of the pair of plates contacts thesecond magnet, and a surface opposed to the first surface of the secondplate contacts a second side of the spacer.

In some implementations, the first side and the second side definetherebetween an interior space, and the device further includes a radiofrequency coil supported on the spacer so as enclose a portion of theinterior space. The radio frequency coil is configured to transmit aradio frequency signal to and receive a radio frequency signal from theinterior space.

In certain implementations, the radio frequency coil has an impedancethat is at least 10 K ohms.

In some implementations, the radio frequency coil is a dual tuned radiofrequency coil configured to be switchable between operation at a firstfrequency and operation at a second frequency.

In certain implementations, the first side of the spacer includes afirst groove and a first support plate disposed in the first groove suchthat an outward facing surface of the first support plate lies flushwith the spacer first side, the second side of the spacer includes asecond groove and a second support plate disposed in the second groovesuch that an outward facing surface of the second support plate liesflush with the spacer second side, and each of the first support plateand the second support plate includes a through opening that isconfigured to receive and support the radio frequency coil within theinterior space.

In some implementations, the first support plate and the second supportplate are formed of a sodium-free plastic.

In certain implementations, the radio frequency coil includes a hollowrectangular form and an electrical conductor that is wound about a coilaxis, and the radio frequency coil is oriented within the spacer suchthat the coil axis is generally parallel to the spacer first side andtransverse to flux lines associated with the magnetic attraction forceof the two magnets.

In some implementations, the spacer is clamped between the magnets dueto the magnetic attractive force.

In certain implementations, the first magnet includes a first polepiece, the second magnet includes a second pole piece, the first side ofthe spacer contacts the first pole piece, and the second side of thespacer contacts the second pole piece.

In some implementations, the spacer includes a spacer body including aspacer body first side and a spacer body second side. The spacer bodydefines an internal space. The spacer also includes a first supportplate that is disposed in a first groove formed in the spacer body firstside and includes a first opening, a second support plate that isdisposed in a second groove formed in the spacer body second side andincludes a second opening that is aligned with the first opening, aradio frequency coil assembly disposed within the first opening and thesecond opening such that a portion of the internal space is enclosed bythe radio frequency coil assembly, a first electrically conductiveshield plate disposed on the spacer body first side, and a secondelectrically conductive shield plate disposed on the spacer body secondside.

In certain implementations, the dialysis system further includes adialysis machine including a compartment, and a module that can bedisposed in the compartment. The module includes the device.

In some implementations, the device further includes a spacer disposedbetween the first magnet and the second magnet. The spacer is configuredto maintain first air gap between the first magnet and the secondmagnet.

In certain implementations, the device further includes a spacerdisposed between the first magnet and the second magnet. The spacer isconfigured to maintain a space between the first magnet and the secondmagnet. The spacer includes a first side that faces the first magnet anda second side that is opposed to the first side and faces the secondmagnet. The spacer has a shape that orients the first magnet relative tothe second magnet in a manner such that a pole face of the first magnetis maintained substantially parallel to a pole face of the secondmagnet.

In some implementations, the spacer includes a spacer body including aspacer body first side and a spacer body second side. The spacer bodydefines an internal space. The spacer further includes a first supportplate that is disposed in a first groove formed in the spacer body firstside and includes a first opening, a second support plate that isdisposed in a second groove formed in the spacer body second side andincludes a second opening that is aligned with the first opening, aradio frequency coil assembly disposed within the first opening and thesecond opening such that a portion of the internal space is enclosed bythe radio frequency coil assembly, a first electrically conductiveshield plate disposed on the spacer body first side, and a secondelectrically conductive shield plate disposed on the spacer body secondside.

In certain implementations, the device further includes a radiofrequency coil disposed in the first air gap between the first magnetand the second magnet. The radio frequency coil is configured totransmit a radio frequency signal to, and receive a radio frequencysignal from, the sample when the sample is disposed within the radiofrequency coil.

In some implementations, the radio frequency coil has an impedance of atleast 10 K ohms.

In certain implementations, the radio frequency coil is a dual tunedradio frequency coil configured to be switchable between operation at afirst frequency and operation at a second frequency.

In some implementations, the main air gap has a dimension correspondingto the distance between the first magnet and the second magnet, and thedimension is in a range of 20 mm to 30 mm.

In certain implementations, the second air gap and the third air gaphave a dimension corresponding to the distance between the first framemember and the second frame member, and the dimension is in a range of0.5 mm to 2.0 mm.

In some implementations, the first magnet includes a first magnetassembly including the first magnet and a first pole piece disposed on apole face of the first magnet, and the second magnet includes a secondmagnet assembly including the second magnet and a second pole piecedisposed on a pole face of the second magnet.

In certain implementations, the dialysis system further includes adialysis machine including a compartment, and a module that can bedisposed in the compartment. The module includes the device formeasuring a concentration of a substance in a sample.

In some implementations, the first member and the second member areformed of metal.

In certain implementations, the first member and the second member areformed of copper.

In some implementations, the first member and the second member have askin depth corresponding to an operating frequency of the radiofrequency coil, and the first member and the second member have athickness that is at least equal to the skin depth, where the thicknesscorresponds to the dimension of the first member and the second memberin a direction parallel to the magnetic field.

In certain implementations, the first member and the second member havea shape that corresponds to the shape of a pole face of the first magnetand the second magnet.

In some implementations, the device further includes a spacer disposedin the space and configured to maintain the space between the firstmagnet and the second magnet. The spacer includes a spacer first sidethat contacts the first magnet and a spacer second side that is opposedto the spacer first side and contacts the second magnet. The spacerfirst side and the spacer second side define therebetween a recess thatis configured to receive the sample.

In certain implementations, the first side of the spacer and the secondside of the spacer are planar, and a plane defined by the first side ofthe spacer is substantially parallel to a plane defined by the secondside of the spacer.

In some implementations, the radio frequency coil has an impedance of atleast 10 K ohms.

In certain implementations, the radio frequency coil is a dual tunedradio frequency coil configured to be switchable between operation at afirst frequency and operation at a second frequency.

In some implementations, the device includes a spacer disposed in thespace and configured to maintain the space between the first magnet andthe second magnet. The spacer includes a first side including a firstgroove and a first support plate disposed in the first groove such thatan outward facing surface of the first support plate lies flush with thespacer first side, and a second side including a second groove and asecond support plate disposed in the second groove such that an outwardfacing surface of the second support plate lies flush with the spacersecond side. Each of the first support plate and the second supportplate includes a through opening that is configured to receive andsupport the radio frequency coil within the space.

In certain implementations, the first support plate and the secondsupport plate are formed of a sodium-free plastic.

In some implementations, the radio frequency coil includes a hollowrectangular form and an electrical conductor that is wound about a coilaxis, and the radio frequency coil is oriented within the spacer suchthat the coil axis is generally parallel to the spacer first side andtransverse to flux lines associated with the magnetic attraction forceof the two magnets.

In certain implementations, the device includes a magnet supportstructure including a first frame member and a second frame member,where the first magnet is supported on the first frame member, thesecond magnet is supported on the second frame member, and the magnetsupport structure supports the first magnet and second magnet in amanner such that the first frame member and second frame membercooperate to substantially surround both the first magnet and the secondmagnet, the first space exists between the first magnet and the secondmagnet, a second space exists between the first frame member and thesecond frame member, and a third space exists between the first framemember and the second frame member at a location spaced apart from thefirst space and the second air space.

In some implementations, the dialysis system further includes a dialysismachine including a compartment, and a module that can be disposed inthe compartment. The module includes the device.

In one aspect of the invention, a medical fluid cartridge includes abody including a first portion defining a first fluid passageway and asecond portion defining a second fluid passageway in fluid communicationwith the first fluid passageway. The body defines a gap between thefirst portion of the body and the second portion of the body such thatthe first portion of the body can be disposed in a radio frequencydevice while the second portion of the body remains outside the radiofrequency device.

In another aspect of the invention, a medical fluid system includes amedical fluid pumping machine and a magnet assembly defining a cavity inwhich a radio frequency device is positioned. The magnet assembly isconfigured to generate a magnetic field within the cavity.

The medical fluid system also includes a medical fluid cartridgeincluding a body having a first portion defining a first fluidpassageway and a second portion defining a second fluid passageway influid communication with the first fluid passageway. The first portionof the body is configured to be disposed within the radio frequencydevice positioned within the cavity of the magnet assembly, and thesecond portion of the body is configured to be disposed within thecavity defined by the magnet assembly and to remain outside the radiofrequency device.

In a further aspect of the invention, a medical fluid cartridge includesa body including a first portion defining a meandering fluid passagewayextending between a fluid inlet port and a fluid outlet port and asecond portion defining a fluid passageway extending between the fluidinlet port and the fluid outlet port. The meandering fluid passagewayand the fluid passageway defined by the second portion are configured sothat when fluid flows into the medical fluid cartridge via the fluidinlet port, a flow rate of fluid flowing through the meandering fluidpassageway is less than a flow rate of fluid flowing through the fluidpassageway defined by the second portion.

In an additional aspect of the invention, a medical fluid systemincludes a medical fluid pumping machine, a magnet assembly defining acavity, and a medical fluid cartridge. The magnet assembly is configuredto generate a magnetic field within the cavity. The medical fluidcartridge includes a body having a first portion defining a meanderingfluid passageway extending between a fluid inlet port and a fluid outletport and a second portion defining a fluid passageway extending betweenthe fluid inlet port and the fluid outlet port. The first portion of thebody is configured to be disposed within the cavity of the magnetassembly, and the meandering fluid passageway and the fluid passagewaydefined by the second portion are configured so that when fluid flowsinto the medical fluid cartridge via the fluid inlet port, a flow rateof fluid flowing through the meandering fluid passageway is less than aflow rate of fluid flowing through the fluid passageway defined by thesecond portion.

In some implementations, the first fluid passageway is a meanderingfluid passageway.

In certain implementations, the first fluid passageway is a U-shapedfluid passageway.

In some implementations, the second fluid passageway is a meanderingfluid passageway.

In certain implementations, the second fluid passageway is a U-shapedfluid passageway.

In some implementations, the first fluid passageway has a flow area thatis greater than a flow area of the second fluid passageway.

In certain implementations, the flow area of the first fluid passagewayis 2 to 10 times greater than the flow area of the second fluidpassageway.

In some implementations, the medical fluid cartridge includes a fluidinlet port via which medical fluid enters the medical fluid cartridgeand a fluid outlet port via which medical fluid exits the medical fluidcartridge.

In certain implementations, the first fluid passageway is configuredsuch that fluid flows through the first fluid passageway at a rate thatis lower than a rate at which the medical fluid flows through the fluidinlet port.

In some implementations, a flow area of the first fluid passageway isgreater than a flow area of the fluid inlet port.

In certain implementations, the first fluid passageway is a meanderingfluid passageway.

In some implementations, the second fluid passageway is configured suchthat fluid flows through the second fluid passageway at a rate that islower than a rate at which the medical fluid flows through the fluidinlet port.

In certain implementations, a flow area of the first fluid passagewayand a flow area of the second fluid passageway are greater than a flowarea of the fluid inlet port.

In some implementations, the first and second fluid passageways areconfigured such that the rate at which fluid flows through the firstfluid passageway is lower than the rate at which the medical fluid flowsthrough the second fluid passageway.

In certain implementations, a flow area of the first fluid passageway isgreater than a flow area of the second fluid passageway.

In some implementations, the first fluid passageway is a meanderingfluid passageway.

In certain implementations, the second fluid passageway is configuredsuch that fluid flows through the second fluid passageway at a rate thatis lower than a rate at which the medical fluid flows through the fluidinlet port.

In some implementations, a flow area of the second fluid passageway isgreater than a flow area of the fluid inlet port.

In certain implementations, the second fluid passageway is a meanderingfluid passageway.

In some implementations, the second fluid passageway is a meanderingfluid passageway.

In certain implementations, the body includes a third portion having athird fluid passageway.

In some implementations, the third fluid passageway is a straight fluidpassageway.

In certain implementations, the medical fluid cartridge includes a fluidinlet port via which medical fluid enters the medical fluid cartridgeand a fluid outlet port via which medical fluid exits the medical fluidcartridge.

In some implementations, the third fluid passageway is configured suchthat fluid flows through the third fluid passageway at a rate that islower than a rate at which the medical fluid flows through the fluidinlet port and that is greater than rates at which the medical fluidflows through the first and second fluid passageways.

In certain implementations, a flow area of the third fluid passageway isgreater than a flow area of the second fluid passageway.

In some implementations, the flow area of the third fluid passageway isat least 5 times greater than the flow area of the second fluidpassageway.

In certain implementations, the flow area of the third fluid passagewayis 5 to 10 times greater than the flow area of the second fluidpassageway.

In some implementations, the first, second, and third fluid passagewaysare configured so that when fluid enters the fluid inlet port, a firstportion of the fluid passes through the second fluid passageway to thefirst fluid passageway and then to the fluid outlet port, and a secondportion of the fluid passes through the third fluid passageway to thefluid outlet port.

In certain implementations, at least one of the first and second fluidpassageways is a meandering fluid passageway.

In some implementations, the body includes a wall that divides thesecond fluid passageway from the third fluid passageway near the fluidinlet port and a wall that divides the first fluid passageway from thethird fluid passageway near the fluid outlet port.

In certain implementations, the body of the medical fluid cartridgeincludes a base and a cover attached to the base, the base and the covercooperating to define the first and second fluid passageways.

In some implementations, the medical fluid cartridge includes indiciathat indicates a volume of the first fluid passageway.

In certain implementations, the indicia is machine-readable.

In some implementations, the indicia includes a barcode.

In certain implementations, the medical fluid cartridge is a dialysisfluid cartridge.

In some implementations, the dialysis fluid cartridge is a bloodcartridge.

In certain implementations, the dialysis fluid cartridge is a dialysatecartridge.

In some implementations, the gap between the first portion of the bodyand the second portion of the body allows the first portion of the bodyto be disposed in a radio frequency coil while the second portion of thebody remains outside the radio frequency coil.

In certain implementations, the magnet assembly is part of the medicalfluid pumping machine.

In some implementations, the magnet assembly is part of a module that isreleasably attached to a housing of the medical fluid pumping machine.

In certain implementations, the magnet assembly includes a pair ofmagnets attached to a frame.

In some implementations, the frame includes two U-shaped members thatcooperate to form the cavity.

In certain implementations, the medical fluid system further includes amedical fluid line connected to the cartridge to carry medical fluid tothe cartridge.

In some implementations, the medical fluid pumping machine includes apump that is operably connected to the medical fluid line to pumpmedical fluid to the cartridge.

In certain implementations, the medical fluid pumping machine is adialysis machine.

In some implementations, the dialysis machine is a hemodialysis machine.

In certain implementations, medical fluid cartridge is removable fromthe cavity of the magnet assembly.

In some implementations, the meandering fluid passageway and the fluidpassageway defined by the second portion are configured so that whenfluid flows into the medical fluid cartridge via the fluid inlet port,the flow rate of fluid flowing through the meandering fluid passagewayis 10 percent to 30 percent of the flow rate of fluid flowing throughthe fluid passageway defined by the second portion.

In certain implementations, a flow area of the fluid passageway definedby the second portion is 5 to 15 times greater than a flow area of themeandering fluid passageway.

In some implementations, the body is configured such that 10 percent to30 percent by volume of the medical fluid flowing through the fluidinlet port passes through the meandering fluid passageway.

In certain implementations, the fluid passageway defined by the secondportion is a substantially straight fluid passageway.

In some implementations, the body is configured such that substantiallyall of the medical fluid that flows through the fluid inlet and thatdoes not pass through the meandering fluid passageway passes through thesubstantially straight fluid passageway.

In certain implementations, the body is configured such that 10 percentto 30 percent by volume of the medical fluid flowing through the fluidinlet port passes through the meandering fluid passageway, and 70percent to 90 percent by volume of the medical fluid flowing through thefluid inlet port passes through the substantially straight fluidpassageway.

In some implementations, the body includes a wall that divides themeandering fluid passageway from the substantially straight fluidpassageway near the fluid inlet port and near the fluid outlet port.

In certain implementations, the meandering fluid passageway is aU-shaped fluid passageway.

In some implementations, the meandering fluid passageway has a firstregion and a second region, a flow area of the first region beinggreater than a flow area of the second region.

In certain implementations, the second region of the meandering fluidpassageway is configured to carry fluid to the first region of themeandering fluid passageway.

In some implementations, the body of the medical fluid cartridgeincludes a base and a cover attached to the base, the base and the covercooperating to define the meandering fluid passageway.

In certain implementations, the medical fluid cartridge includes indiciathat indicates a volume of a first region of the meandering fluidpassageway.

In some implementations, the indicia is machine-readable.

In certain implementations, the indicia includes a barcode.

In some implementations, the medical fluid cartridge is a dialysis fluidcartridge.

In certain implementations, the dialysis fluid cartridge is a bloodcartridge.

In some implementations, the dialysis fluid cartridge is a dialysatecartridge.

In certain implementations, the meandering fluid passageway and thefluid passageway defined by the second portion are configured so thatwhen fluid flows into the medical fluid cartridge via the fluid inletport, the flow rate of fluid flowing through the meandering fluidpassageway is 10 percent to 30 percent of the flow rate of fluid flowingthrough the fluid passageway defined by the second portion.

In some implementations, a flow area of the fluid passageway defined bythe second portion is 5 to 15 times greater than a flow area of themeandering fluid passageway.

In certain implementations, the body is configured such that 10 percentto 30 percent by volume of the medical fluid flowing through the fluidinlet port passes through the meandering fluid passageway.

In some implementations, the fluid passageway defined by the secondportion is a substantially straight fluid passageway.

In certain implementations, the medical system further includes a radiofrequency device disposed in the cavity of the magnet assembly, and thefirst portion of the body of the medical fluid cartridge is configuredto be disposed in the radio frequency device.

In some implementations, a second portion of the body of the medicalfluid cartridge is configured to be disposed within the cavity definedby the magnet assembly and to remain outside the radio frequency devicewhen the first portion of the body of the medical fluid cartridge isdisposed in the radio frequency device.

In certain implementations, the radio frequency device is a radiofrequency coil.

In some implementations, the magnet assembly is part of the medicalfluid pumping machine.

In certain implementations, the magnet assembly is part of a module thatis releasably attached to a housing of the medical fluid pumpingmachine.

In some implementations, the magnet assembly includes a pair of magnetsattached to a frame.

In certain implementations, the frame includes two U-shaped members thatcooperate to form the cavity.

In some implementations, the medical fluid system further includes amedical fluid line connected to the cartridge to carry medical fluid tothe cartridge.

In certain implementations, the medical fluid pumping machine includes apump that is operably connected to the medical fluid line to pumpmedical fluid to the cartridge.

In some implementations, the medical fluid pumping machine is a dialysismachine.

In certain implementations, the dialysis machine is a hemodialysismachine.

In some implementations, the medical fluid cartridge is removable fromthe cavity of the magnet assembly.

Implementations can include one or more of the following advantages.

Many of the sensor assemblies described herein are designed to carry outonline measurements of the concentration of a substance in medical fluidas the medical fluid is being pumped through a medical fluid circuitduring a medical treatment. This online measurement technique allowssubstance concentrations falling outside a desired concentration rangeto be detected quickly and to be quickly remedied. In certainimplementations, the sensor assembly is used to carry out onlinemeasurements of a sodium concentration in a patient's blood duringhemodialysis treatment. These measurements can be used to quicklydetermine when the sodium concentration of the patient's blood fallsoutside of a desired concentration range and to quickly adjust thepatient's blood sodium concentration when such a condition occurs. Forexample, in response to determining that the sodium concentration of thepatient's blood falls outside of a desired concentration range, thesodium content in the dialysate can be adjusted to cause an adjustmentof the sodium concentration of the patient's blood. Quickly adjustingthe blood sodium concentration in this manner can prevent the patient'sblood sodium concentrations from falling outside a desired concentrationrange for a significant period of time and can thus reduce thelikelihood of the patient feeling discomfort resulting from a bloodsodium concentration that is too high or too low.

Many of the sensor assemblies described herein can be used to determinethe concentration of a substance in a medical fluid with greateraccuracy than sensors, such as conductivity sensors, that havetraditionally been used to determine the concentration of a substance ina medical fluid. In some implementations, the sensor assemblies canmeasure the blood sodium to an accuracy of less than +/3 mM with ameasurement time of only a few minutes. As a result, systems employingmany of the sensor assemblies described herein can monitor and maintainthe concentration of the substance in the medical fluid with greateraccuracy than many conventional systems. In implementations in which thesensor assemblies are used in hemodialysis systems to measure the sodiumconcentration in a patient's blood, the increased accuracy of the sensorcan ensure that the patient's blood sodium levels are maintained withina desired concentration range with greater accuracy, which reduces thelikelihood of the patient feeling discomfort resulting from a bloodsodium concentration that is too high or too low.

Advantageously, many of the sensor assemblies described herein can beimplemented for a much lower cost than some conventional spectroscopicanalysis devices including nuclear magnetic resonance (NMR) technology.This can be achieved in part due to the type of measurement made usingthe sensor assemblies described herein. In many conventionalspectroscopic NMR devices, a very strong and homogeneous magnetic fieldis required for resolving very tiny frequency differences between atoms,which in turn requires relatively large and expensive magnets. Since thesensor assemblies described herein are typically used to determine anamount of only one or two particular substances in a sample, themagnetic field requirements of the sensor assemblies are less stringentthan some conventional spectroscopic NMR devices. Cost reductions canalso be obtained because the sensor assemblies described hereintypically do not require electronic or mechanical shimming to obtain asufficiently homogeneous magnetic field.

In some implementations, a relatively simple and lightweight structurepermits the sensor assembly to provide a desired level of magnetic fieldhomogeneity by arranging two magnets so that their respective pole facesare spaced apart and parallel. In certain implementations, a spacer ispositioned between the magnets to ensure that the respective magnet polefaces are parallel to a sufficient tolerance. Since the spacer, ratherthan the support structure, is relied upon to ensure that the pole facesof the magnets are parallel, the support structure can be simple andlightweight relative to many conventional NMR magnet support structures,such as many conventional C-core magnet support structures.

In certain implementations, the sensor assembly includes a spacerassembly having shield plates to address eddy currents that tend to formin pole pieces of the magnet assembly during operation of the RF deviceand can in turn result in acoustic ringing in a gap between magnets ofthe magnet assembly. The shield plates are formed of a non-magnetic,electrically conductive material (e.g., copper). Since the shield platematerial is not magnetic, the shield plates do not become saturated. Inaddition, since the shield plate material is electrically conductive,the shield plate skin depth tends to be small, which permits the use ofthin plates and reduces or minimizes the size of the overall assembly.In addition, the small skin depth can result in lower electrical losses,and can thus increase the sensitivity of the RF device.

In some implementations, the sensor assembly includes an isolationcircuit that can be switched between a transmitting mode (when the RFdevice is transmitting RF energy) and a receiving mode (when the RFdevice is receiving RF energy). In the transmitting mode, receivingcomponents are isolated from high voltages associated with thetransmission of the RF energy. Similarly, in the receiving mode,transmitting components are isolated from low voltage receivingcomponents. This arrangement allows low voltage electronic components tobe placed on the circuit without risk of damage.

Some of the medical fluid cartridges described herein are designed toslow the flow of medical fluid through the cartridge and/or to lengthena flow path of medical fluid through the cartridge. The slower flow rateand/or lengthened flow path can increase or maximize the amount of timethat the medical fluid spends flowing through the magnet assembly (i.e.,the magnetic field generated by the magnet assembly) and the RF deviceof the system. Thus, this cartridge design permits the use of a magnetassembly that is smaller and less expensive than magnet assemblies foundin many traditional NMR sensors. The reduced size and cost of the magnetassembly makes it cost-feasible to use NMR sensor assemblies of the typedescribed herein in medical fluid pumping machines (e.g., hemodialysismachines).

In certain implementations, the cartridge includes a fluid passagewaythat directs medical fluid through a magnetic field generated by thesystem before the medical fluid reaches a portion of the cartridge inwhich RF energy is applied to and received from the medical fluid. Forexample, the fluid passageway can be a meandering fluid passageway thatis positioned within the magnetic field generated by the system butoutside the RF device (e.g., an RF coil) of the system during use. Thedesign of this fluid passageway can help to ensure that the medicalfluid travels within the magnetic field for a sufficient period of timeto polarize atoms (e.g., sodium atoms) of the medical fluid, which helpsto ensure that those atoms can be accurately detected and counted as themedical fluid passes through the portion of the cartridge in which theRF energy is applied to and received from the medical fluid.

In certain implementations, the cartridge includes a fluid passagewaythat bypasses the fluid passageway through which the medical fluid to beanalyzed flows. This bypass fluid passageway can be straight and canhave a greater flow area than the fluid passageway carrying the medicalfluid to be tested. This arrangement can ensure that a significantvolume (e.g., at least 70 percent by volume) of the medical fluidentering the cartridge is allowed to pass through the cartridge at ahigh flow rate. The straightness of the bypass fluid passageway and thehigh flow rate of the medical fluid passing through the bypass fluidpassageway can help to ensure that the medical fluid passes through thecartridge without damage to the medical fluid. In implementations inwhich the medical fluid is blood, for example, the design of the bypassfluid passageway can help to ensure that the blood flows through thecartridge without clotting and without damage to the various componentsof the blood. The high flow rate of the medical fluid passing throughthe bypass fluid passageway can also allow the medical fluid pump of thesystem to be operated at a constant speed without placing excessiveburden on the medical fluid pump.

In some implementations, rather than detecting the concentration of asubstance in a medical fluid while the medical fluid is flowing throughthe cartridge, the medical fluid is diverted to a chamber of a cartridgewhere the medical fluid remains static or stagnant while being tested.Testing a static sample of medical fluid in this way can increase theease with which the concentration of a substance in the medical fluidcan be determined. For example, it is unnecessary to account for a flowrate of the medical fluid when determining the concentration of thesubstance when the sample is static. In certain cases, testing a staticsample of medical fluid in this way may result in a more accuratedetermination of the concentration of the substance in the medicalfluid.

In certain implementations, sensor assemblies described herein can beused to indirectly determine the concentration of a substance (e.g.,sodium) in the blood of a patient. For example, the sensor assembly canbe positioned along a dialysate circuit of a hemodialysis system tomeasure the concentration of sodium in the dialysate. By comparing theconcentration of sodium in the fresh dialysate (i.e., the dialysate thathas not yet passed through a dialyzer of the system along with thepatient's blood) to the concentration of sodium in the spent dialysate(i.e., the dialysate that has passed through the dialyzer along with thepatient's blood), it is possible to determine the concentration ofsodium in the patient's blood. This technique permits sodium levels inthe patient's blood to be determined without diverting any blood fromthe extracorporeal blood line set of the hemodialysis system to a bloodcartridge. As a result, this technique can make it easier to maintain adesired constant blood flow rate within the extracorporeal blood lineset. Further, the samples of fresh and spent dialysate that are analyzedin this manner can be static samples, which can increase the ease, andin some cases the accuracy, with which the concentration of sodiumwithin the fresh and spent dialysate is determined.

In implementations in which samples of fresh and spent dialysate areanalyzed to indirectly determine the concentration of a substance (e.g.,sodium) in a patient's blood, the medical fluid cartridge into which thefresh and spent dialysate samples are directed for analysis can bereusable from patient to patient since the dialysate delivered to themedical fluid cartridge does not come into contact with a patient'sblood. This allows a relatively expensive, precisely machined medicalfluid cartridge to be used, which can permit accurate concentrations ofsubstances in the dialysate to be determined without having to accountfor variations in the volume of one medical fluid cartridge to the nextor discrepancies between the actual volume of the medical fluidcartridge and the intended volume of the medical fluid cartridge.

In certain implementations, a volume of the medical fluid cartridge(e.g., a volume of the portion of the fluid passageway of the medicalfluid cartridge that holds the medical fluid to which RF energy isapplied and from which RF energy is received during analysis) isdetermined with a high level of accuracy and this volume can be used toaccurately determine the concentration of a substance in the medicalfluid. This technique permits the medical fluid cassette, which may be asingle-use disposable component, to be formed with a lower precision(e.g., a precision typical of injection molding), which allows themedical fluid cassette to be manufactured in a cost feasible mannerwithout significantly impacting the accuracy with which theconcentration of the substance in the medical fluid can be determined.For example, the actual volume of the medical fluid cassette may differslightly from the intended volume and, in the case of mass production ofsuch medical fluid cartridges, the actual volume of one medical fluidcartridge to the next may differ slightly without significantlyimpacting the concentration detected by the sensor assembly when usedwith those medical fluid cartridges since the actual volume of eachmedical fluid cartridge will be determined and used to determine theconcentration of the substance in the medical fluid.

In some implementations, the sensor assembly is used to detect aconcentration of a substance (e.g., sodium) in a reference fluid (e.g.,a reference fluid contained in a reference fluid cartridge) prior todetecting the concentration of that substance in the medical fluid inthe medical fluid cartridge. The actual concentration of the substancein the reference fluid is known. Therefore, the concentration of thesubstance in the reference fluid that is detected by the sensor assemblycan be used to calibrate the sensor assembly. This is advantageous sincethe sensitivity of the sensor assembly may change from treatment totreatment due to changing conditions, such as slight changes in theuniformity of the magnetic field generated by the sensor assembly (i.e.,generated by the magnet assembly of the sensor assembly), trace amountsof the substance to be measured (e.g., sodium) in the system, etc.

In some implementations, hydrogen and a second substance (e.g., sodium)are detected in a reference fluid in a reference fluid cartridge.Hydrogen and the second substance are also detected in a saline solutionand a medical fluid sample (e.g., a blood sample), respectively, in amedical fluid cartridge. The detected readings of the quantities orconcentrations of the hydrogen and the second substance in the referencefluid, saline solution, and medical fluid sample can then be compared toaccurately determine the concentration of the second substance in themedical fluid sample. For example, the ratio of the detected reading ofthe second substance to the detected reading of hydrogen in thereference fluid can be compared to the ratio of the detected reading ofthe second substance in the medical fluid sample to the detected readingof hydrogen in the saline solution and that information can be used toaccurately determine the concentration of the second substance in themedical fluid. Advantageously, such a technique can be used to determinethe concentration of the second substance in the medical fluid samplewithout knowing the volume of the medical fluid cartridge in which themedical fluid sample resides when analyzed because the hydrogenconcentrations in the reference fluid and the medical fluid are the sameand the ratios of the detected reading of the second substance to thedetected reading of hydrogen in the fluids are not dependent on thevolumes of the cartridges. In other words, a difference between thosedetermined ratios can be attributed to a difference in theconcentrations of the second substance in the reference fluid and themedical fluid sample and can be used to determine the actualconcentration of the second substance in the medical fluid sample.

In certain implementations, a reference cartridge containing a referencefluid of the type described above is provided with the sensor assembly.In addition to allowing the sensor assembly to be calibrated in themanner discussed above, the reference fluid cartridge can help toprevent damage to the magnet assembly. For example, the reference fluidcartridge can be stored within the magnet assembly between uses and canthus reduce the risk of unintended objects (e.g., magnetic objects)being inserted into and trapped within the magnet assembly.

Other aspects, features, and advantages will be apparent from thedetailed description, the drawings, and the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a front view of a hemodialysis machine including a nuclearmagnetic resonance (NMR) module mounted in a mid-section of the machine.

FIG. 2 is an enlarged view of the midsection of the hemodialysis machineof FIG. 1.

FIG. 3 is a perspective view of the NMR module of FIG. 1 isolated fromthe dialysis machine. A cover plate of the NMR module has been omittedto better illustrate an internal NMR sensor assembly of the NMR module.

FIG. 4 is an enlarged view of the region 4 in FIG. 3, illustrating ablood cartridge disposed in the NMR sensor assembly.

FIG. 5 is a perspective view of a support frame and magnet units of theNMR sensor assembly of FIG. 3.

FIG. 6 is an exploded perspective view of the support frame, the magnetunits, and a spacer assembly of the NMR sensor assembly of FIG. 3.

FIG. 7 is a front view of the support frame and the magnet units of theNMR sensor assembly of FIG. 3 (with the spacer assembly omitted),illustrating a magnetic loop formed therein.

FIG. 8 is an exploded perspective view of the spacer assembly of the NMRsensor assembly of FIG. 3 in combination with a radio frequency (RF)coil assembly of the NMR sensor assembly of FIG. 3.

FIG. 9 is a top view of the spacer assembly and RF coil assembly of theNMR sensor assembly of FIG. 3 with shield plates of the spacer assemblyomitted to better illustrate interior components of the spacer assembly.

FIG. 10 is a perspective view of the spacer assembly and RF coilassembly of the NMR sensor assembly of FIG. 3 (with the shield plates ofthe spacer assembly omitted), illustrating an opening through which aportion of the blood cartridge can be inserted into an internal space ofthe spacer assembly and the RF coil assembly.

FIG. 11 is a perspective cross-sectional view of the spacer assembly andRF coil assembly along line 11-11 in FIG. 9, illustrating the RF coilassembly supported by support plates so as to surround a portion of thespacer assembly internal space into which a portion of the bloodcartridge can be inserted during use.

FIG. 12 is a perspective view of a first side of a spacer body of thespacer assembly of FIG. 6.

FIG. 13 is a perspective view of a second side of the spacer body of thespacer assembly of FIG. 6.

FIG. 14 is a perspective view of the first side of the spacer body ofthe spacer assembly of FIG. 6, illustrating support plates assembledwith the spacer body.

FIG. 15 is a perspective view of the second side of the spacer body ofthe spacer assembly of FIG. 6, illustrating support plates assembledwith the spacer body.

FIG. 16 is a perspective view of the RF coil assembly of the NMR sensorassembly of FIG. 3.

FIG. 17 is an exploded perspective view of the RF coil assembly of FIG.16.

FIG. 18 is a perspective cross-sectional view of the RF coil assemblywhen assembled within the NMR sensor assembly of FIG. 3, illustratingthe transverse orientation of the RF coil axis relative to the magneticfield B0 generated by the magnet units.

FIG. 19 shows a block diagram of a circuit used to operate the NMRsensor assembly of FIG. 3.

FIGS. 20 and 21 show a circuit implementing a portion of thefunctionality of the circuit shown in FIG. 19, where FIG. 20 illustratesthe circuit in a receiving mode, and FIG. 21 illustrates the circuit ina transmitting mode.

FIG. 22 is a perspective view of the blood cartridge that can be usedwith the NMR sensor assembly of FIG. 3.

FIG. 23 is a perspective cross-sectional view of the blood cartridge asseen along line 23-23 in FIG. 22.

FIG. 24 is a perspective cross-sectional view of the blood cartridge asseen along line 24-24 in FIG. 22.

FIG. 25 is a perspective cross-sectional view of the blood cartridge asseen along line 25-25 in FIG. 22.

FIG. 26 is a perspective cross-sectional view of the blood cartridge asseen along line 26-26 in FIG. 22.

FIG. 27 is a plan view of the base of the blood cartridge of FIG. 22.

FIG. 28 is an exploded perspective view of the blood cartridge of FIG.22.

FIG. 29 is a perspective view of the blood cartridge of FIG. 22 disposedin the spacer assembly and RF coil assembly of the NMR sensor assemblyof FIG. 3. The shield plates of the spacer assembly have been omitted toexpose certain interior components of the spacer assembly.

FIG. 30 is a perspective cross-sectional view of the cartridge, thespacer assembly, and the RF coil assembly as seen along line 30-30 inFIG. 29.

FIG. 31 is an exploded perspective view of a reference fluid cartridgethat can be used with the NMR sensor assembly of FIG. 3.

FIG. 32 is a schematic illustration of the blood and dialysate circuitsof the hemodialysis system of FIG. 1, showing, among other things, theblood cartridge of FIG. 22 disposed in the NMR sensor assembly of FIG. 3along the blood circuit.

FIG. 33 shows a block diagram of a circuit used to operate an NMR sensorassembly that includes a dual tuned RF coil.

FIG. 34 shows a circuit implementing a portion of the functionality ofthe circuit shown in FIG. 33.

FIG. 35 shows a graph of the frequency response as measured at a firstpoint of the circuit shown in FIG. 34.

FIG. 36 shows a graph of the frequency response as measured at a secondpoint of the circuit shown in FIG. 34.

FIG. 37 shows a graph of the frequency response as measured at a thirdpoint of the circuit shown in FIG. 34.

FIG. 38 shows a graph of the frequency response of the dual tuned RFcoil of FIG. 33.

FIG. 39 is a schematic illustration of the blood and dialysate circuitsof another hemodialysis system, including a dialysate cartridge disposedin an NMR sensor assembly positioned along the dialysate circuit.

FIG. 40 is a perspective cross-sectional view of the dialysate cartridgeof FIG. 39 disposed in a spacer assembly and RF coil of the NMR sensorassembly of FIG. 39.

FIG. 41 is a flow chart illustrating a method of determining aconcentration of a substance in a medical fluid.

FIG. 42 is a flow chart illustrating another method of determining aconcentration of a substance in a medical fluid.

FIG. 43 is a flow chart illustrating an additional method of determininga concentration of a substance in a medical fluid.

FIG. 44 is a flow chart illustrating a method of marking a medical fluidcartridge with machine readable indicia.

DETAILED DESCRIPTION

In general, this disclosure relates to sensor assemblies (e.g., nuclearmagnetic resonance (NMR) sensor assemblies) that can be used to detectthe concentration of a substance (e.g., sodium and/or hydrogen) in amedical fluid (e.g., blood and/or dialysate). Examples of such sensorassemblies and related systems and methods are described herein.

Referring to FIG. 1, a hemodialysis system 101 includes a hemodialysismachine 100 having an NMR module 138 that can be used to determine theconcentration of sodium in blood of a patient during hemodialysistreatment. The system 101 can be used to match the concentration ofsodium in dialysate used to perform the hemodialysis treatment to thatof the patient's blood and/or to adjust the concentration of sodium inthe blood if the sodium concentration detected by the NMR module 138falls outside a desired range. The NMR module 138, methods of using theNMR module 138 to detect the concentration of sodium in blood, andvarious other components of the dialysis system 101 will be described indetail below.

Still referring to FIG. 1, the hemodialysis machine 100 includes adisplay 118 (which may include a touch screen) and a control panel 112,whereby operator selections and instructions can be input to and storedby a control unit of the hemodialysis machine 100. The hemodialysismachine 100 also includes modules that house components used to performhemodialysis, including a blood pump module 132, a heparin pump module134, an air release device and level detector module 136, and the NMRmodule 138.

In use, a disposable blood line set 140, which forms a blood circuit ofthe system 101, is connected to the modules 132, 134, 136, 138 on thefront side of the hemodialysis machine 100. During treatment, patientlines 1106, 1108 of the blood line set 140 are connected to the patientand a pump tubing 1160 of the blood line set 140 is connected to theblood pump 1132. As the blood pump 1132 is operated, blood is drawn fromthe patient, pumped through the dialyzer 1110, and then returned to thepatient.

Referring also to FIG. 2, which illustrates the mid section of thehemodialysis machine 100 without the blood line set 140 connected to themodules 132, 134, 136, 138, the first portion 142 of the blood line set140 includes pump tubing 1160 that is connected to the blood pump module132 in a manner so as to operatively engage a peristaltic blood pump1132 of the blood pump module 132. Operation of the blood pump 1132pumps blood through the blood line set 140.

A drug delivery line 1174 of the blood line set 140 intersects the firstportion 142 at a location between the blood pump 1132 and the dialyzer1110, and is connected a syringe 1178. The syringe 1178 is connected toa syringe pump 1192 of the heparin pump module 134. The heparin pumpmodule 134 also includes a bracket 135 to hold the syringe 1178 in thesyringe pump 1192. With the syringe 1178 held in the bracket 135 in thismanner, the syringe pump 1192 can be operated to move a plunger of thesyringe 1178 and thus eject liquid from the syringe 1178 through thedrug delivery line 1174. The heparin pump module 134 can thus be used toinject heparin from the syringe 1178 into the blood circuit via the drugdelivery line 1174 during a hemodialysis treatment.

The second portion 144 of the blood line set 140 includes a bloodcartridge 500 (shown in FIGS. 1 and 3) connected in series with bloodlines of the blood line set 140. The blood cartridge 500 is disposed inan NMR sensor assembly 200 (shown in FIG. 3) of the NMR module 138. Theblood cartridge 500 can be inserted into the NMR assembly 200 via anopening 137 formed in a cover plate 139 of the NMR module 138. The coverplate 139 can help to prevent the NMR sensor assembly 200 disposedwithin the housing of the NMR module 138 from becoming damaged duringuse. As discussed in detail below, the NMR sensor assembly 200 can beused to determine the concentration of sodium in a sample of bloodflowing through the blood cartridge 500 during dialysis treatment.

The second portion 144 of the blood line set 140 also includes an airrelease device (or drip chamber) 1112 at a location downstream fromcassette 500. The air release device 1112 permits gas, such as air, inthe blood to escape before the filtered blood is returned to thepatient. The air release device 1112 can be secured to level detectormodule 136 so as to align with a level detector 1182 that is adapted todetect the level of blood within the air release device 1112.

Referring to FIGS. 3 and 4, the NMR module 138 includes a module housing150 that is configured to be received within a module compartment of thehemodialysis machine 100. The NMR sensor assembly 200 is mounted withinthe housing 150. In some implementations, the module housing 150includes a Faraday cage that is disposed between the housing 150 and theNMR sensor assembly 200.

The NMR sensor assembly 200 includes a pair of magnet units 260, 280supported by a support frame 201 within the housing 150, a spacerassembly 400 that is disposed between the magnet units 260, 280, and aradio frequency (RF) energy transmitting/receiving coil assembly 300that is supported by the spacer assembly 400 so as to be located withina magnetic field B0 between the magnet units 260, 280. As shown in FIG.4, during use, the cartridge 500 of the blood line set 140 is insertedinto the NMR module 138 in a manner such that a fluid passageway withinthe cartridge 500 (i.e., fluid passageway 524 shown in FIG. 27) isdisposed in the magnetic field B0 and within the coil assembly 300. Thecoil assembly 300 transmits RF energy to, and receives RF energy from,blood flowing within the fluid passageway to determine a concentrationof a substance of interest (e.g., sodium) in the blood.

Referring to FIGS. 5 and 6, the support frame 201 is configured tosupport the first magnet unit 260 and the second magnet unit 280 withinthe NMR sensor assembly 200, and is formed by two separate pieces thatinclude a first frame member 202 and a second frame member 232.

The first frame member 202 has a U-shape that includes a first framebase 204, a first frame arm 206 that extends from one end of the firstframe base 204 in a direction perpendicular to the first frame base 204,and a second frame arm 208 that extends from the opposed end of thefirst frame base 204 in a direction perpendicular to the first framebase 204. The first magnet unit 260 is supported by the first framemember 202. In particular, a first pole end 262 of the first magnet 261is secured to the first frame base 204 at a location midway between thefirst frame arm 206 and the second frame arm 208. The first magnet unit260 is disposed on an inside portion 214 of the U-shaped first framemember 202.

The second frame member 232 has a U-shape that includes a second framebase 234, a third frame arm 236 extending from one end of the secondframe base 234 in a direction perpendicular to the second frame base234, and a fourth frame arm 238 extending from the opposed end of thesecond frame base 234 in a direction perpendicular to the second framebase 234. The second magnet unit 280 is supported by the second framemember 232. In particular, a pole end 282 of the second magnet 281 issecured to the second frame base 234 at a location midway between thethird frame arm 236 and the fourth frame arm 238. The second magnet unit280 is disposed on an inside portion 244 of the U-shaped second framemember 232.

The first frame member 202 and the second frame member 232 are formed ofsteel (or one or more other ferromagnetic materials), and the first andsecond magnet units 260, 280 are secured to the support frame 201 viamagnetic attraction.

The first magnet unit 260 includes a first rectangular magnet 261 havinga first end 262 that is connected to the first frame member 202, and asecond end 264 opposed to the first end 262. The first end 262 andsecond end 264 of the first magnet 261 correspond to the poles of thefirst magnet 261. The first magnet unit 260 also includes a softmagnetic pole piece 266 disposed on the first magnet second end 264.Similarly, the second magnet unit 280 includes a second rectangularmagnet 281 having a first end 282 that is connected to the second framemember 232, and a second end 284 opposed to the first end 282. The firstend 282 and second end 284 of the second magnet 281 correspond to thepoles of the second magnet 281. The second magnet unit 280 also includesa soft magnetic pole piece 286 disposed on the second magnet second end284.

The first magnet 261 and the second magnet 281 are permanent magnets. Insome implementations, the first magnet 261 and the second magnet 281 areNdFeB magnets (e.g., formed of an alloy of Neodymium, Iron and Boron).Typically, the magnets 261, 281 are each 70 mm×70 mm×15 mm andmagnetized to about 1.2 Tesla.

The pole pieces 266, 286 are typically formed of a material having highmagnetic permeability, such as soft iron, and serve to direct themagnetic field generated by the magnets 261, 281. The pole pieces 266,286 have a truncated pyramid shape. Thus, the first pole piece 266 has afirst face 268 corresponding to a major base of the truncated pyramidand an opposed, second face 270 corresponding to the minor base of thetruncated pyramid. The first face 268 of the first pole piece 266 hasthe same shape and dimension as, and is aligned with a periphery of, thesecond end 264 of the first magnet 261. Likewise, the second pole piece286 has a first face 288 corresponding to a major base of the truncatedpyramid and an opposed, second face 290 corresponding to the minor baseof the truncated pyramid. The first face 288 of the second pole piece286 has the same shape and dimension as, and is aligned with a peripheryof, the second end 284 of the magnet second magnet 281. The first polepiece 266 and the second pole piece 286 are secured to the respectivemagnet second ends 264, 284 via magnetic attraction.

Referring to FIGS. 5-7, the first frame member 202 and the second framemember 232 cooperate to provide a structure that substantially surroundsthe magnet units 260, 280. More specifically, the magnet units 260, 280are surrounded except for small secondary air gaps g₁, g₂ between endsof the frame members 202, 232, as discussed below. The first magnet unit260 and the second magnet unit 280 are arranged and supported by thesupport frame 201 in a manner such that a main air gap g_(m) existsbetween the first magnet unit 260 and the second magnet unit 280. Inaddition, the first frame member 202 is arranged relative to the secondframe member 232 in a manner such that a free end 210 of the first framearm 206 faces a free end 240 of the third frame arm 236 and is spacedapart from the third frame arm 236 so that the secondary air gap g1exists between the first frame arm 206 and the third frame arm 236.Similarly, a free end 212 of the second frame arm 208 faces a free end242 of the fourth frame arm 238 and is spaced apart from the fourthframe arm 238 so that the other secondary air gap g2 exists between thesecond frame arm 208 and the fourth frame arm 238. The frame members202, 232 are maintained in the spaced apart relationship by the spacerassembly 400 (shown in FIGS. 5 and 6, omitted in FIG. 7 for simplicity).

In some examples, the main air gap g_(m) is significantly larger thanthe secondary air gaps g₁, g₂. In particular, the main air gap g_(m) isdimensioned to correspond to a width of the spacer assembly 400. Thesecondary air gaps g₁ and g₂ are substantially equal in length, and aretypically in a range of about 0.5 mm to 1.0 mm. The presence of thesecondary air gaps g₁, g₂ ensures that the spacer assembly 400, ratherthan the support frame 201, controls the orientation of the first magnetunit 260 relative to the second magnet unit 280. For example, thepresence of the secondary air gaps g₁, g₂ reduces the number ofmechanical tolerances that must be accounted for when manufacturing andassembling the support frame 201 and its accompanying components.

Still referring to FIG. 7, the first magnet unit 260 and the secondmagnet unit 280 are arranged and supported by the support frame 201 in amanner such that an attractive uniform magnetic field B0 having fluxlines extending normal to the faces 270, 290 of the pole pieces 266, 286is formed between the respective magnet units 260, 280. In someimplementations, the magnetic field B0 has a magnetic field strength ina range of about 0.8 to 1.2 Tesla. It is advantageous for the magneticfield homogeneity to be less than +/−0.1 percent, which can be achievedat least in part by ensuring that the width of the main air gap g_(m)between the pole pieces 266, 286 varies by less than +/−0.1 percent. Insome implementations, the width of the main air gap g_(m) is 12.5mm+/−12.5 μm.

A magnetic loop (indicated by the arrows in FIG. 7) is formed thatpasses through the second magnet unit 280, then passes across the mainair gap gm and through the first magnet unit 260 to the first supportmember 202, where it divides and passes through both secondary air gapsg1 and g2 into the second support member 232, and then returns to thesecond magnet unit 280. Although a flux direction is illustrated by theuse of arrows in FIG. 7, the magnetic loop may also be illustrated usingarrows oriented in a direction opposed to the arrows shown in FIG. 7.

Referring briefly again to FIGS. 5 and 6, to maintain the desiredspacing between the first magnet unit 260 and the second magnet unit 280in the presence of the attractive magnetic field B0, and also to ensurethat the free pole end 264 of the first magnet 261 is oriented parallelto the free pole end 284 of the second magnet 281, the spacer assembly400 is positioned between the first magnet unit 260 and the secondmagnet unit 280.

Referring to FIGS. 8-15, the spacer assembly 400 includes a generallyrectangular spacer body 402, a first support plate 460 supported on afirst side 404 of the spacer body 402, and a second support plate 480supported on a second side 406 of the spacer body 402. The spacer body402, the first support plate 460 and the second support plate 480cooperate to define an internal space 420 within the spacer body 402.The RF coil assembly 300 is supported by the first and second supportplates 460, 480 so as to enclose a portion of the internal space 420.The spacer assembly 400 also includes a pair of electrically conductive,non-magnetic shield plates 490, 496. Each of these components of thespacer assembly 400 is described in detail below.

Referring to FIGS. 8-13, the second side 406 of the spacer body 402 isopposed to the first side 404. The spacer body 402 is precisely formed(e.g., using a precision machining process) so that the first side 404and the second side 406 are substantially parallel to one another. Insome implementations, for example, they are angled by no more than 0.2degrees (e.g., 0.1 to 0.2 degrees, 0.14 degrees) relative to oneanother. The spacer body 402 includes a first edge 412, a second edge414 adjoining the first edge 412, a third edge 416 adjoining the secondedge 414 and located on an opposed side of the spacer body 402 relativeto the first edge 412, and a fourth edge 418 that adjoins the first andthird edges 412, 416 and is located on an opposed side of the spacerbody 402 relative to the second edge 414. The first, second, third andfourth edges 412, 414, 416, 418 have a dimension corresponding to thethickness of the spacer body 402 (e.g., the distance between the spacerfirst side 404 and the spacer second side 406).

The first side 404 of the spacer body 402 includes a first side groove408 having a shape that generally corresponds to the shape of the firstsupport plate 460 and a depth that corresponds to the thickness of thefirst support plate. Similarly, the second side 406 of the spacer body402 includes a second side groove 410 having a shape that generallycorresponds to the shape of the second support plate 480 and a depththat corresponds to the thickness of the second support plate 480. Boththe first side groove 408 and the second side groove 410 open at thespacer body fourth edge 418.

The spacer body 402 includes an opening 430 in the first edge 414 thatcommunicates with the first side groove 408 and the second side groove410 to form the internal space 420 within the spacer body 402. Theopening 430 is dimensioned to permit a portion of the blood cartridge500 (shown in FIG. 4) to be inserted into the internal space 420. Theedges 430 a of the opening 430 are beveled to facilitate insertion ofthe cartridge 500.

The spacer body 402 includes a flange 422 that protrudes outward fromthe body second edge 414 in a direction normal to the body second edge414. The flange 422 includes through holes 424 that are dimensioned toreceive fasteners (e.g., bolts), whereby a printed circuit board 350(shown in FIG. 3) including RF coil driving electronics can be securedto the spacer body 402 via the flange 422 in a location that is near theRF coil assembly 300.

The spacer body 402 is formed of a ceramic material. In someimplementations, the spacer body 402 is formed of a machinable glassceramic, for example, Macor® manufactured by Corning, Inc. of Corning,N.Y. Forming the spacer body 402 of a ceramic material is advantageoussince ceramics are well suited for the highly-precise machining requiredto provide a structure having opposed sides that are parallel to therequired extent. In addition, ceramic materials are non-magnetic andnon-electrically conductive. These properties are beneficial for thespacer body 402 when assembled in the gap between the magnet units 260,280, since these properties reduce undesirable acoustic effectsassociated with placement of metal structures in the gap.

FIGS. 9-11, 14 and 15 show the first support plate 460 and the secondsupport plate 480 mounted to the spacer body 402. The first and secondsupport plates 460, 480 are substantially similar in form and function,so only the first support plate 460 will be described here. The firstsupport plate 460 is formed of a sodium-free plastic, such as Acetal,Delrin, polycarbonate, acrylic, or another sodium-free plastic with highdimensional stability. The first support plate 460 is configured to bepress fit within the corresponding groove 408 formed in the spacer body402 such that outward facing surfaces of the first support plate 460 lieflush with the outer surface of the spacer body 402. More specifically,the first support plate 460 lies flush with the first side 404 and thefourth edge 414 of the spacer body 402. The first support plate 460includes a through opening 468 that is shaped and dimensioned to receiveand support portions of the RF coil assembly 300. For example, thethrough opening 468 is generally rectangular in shape and includesinwardly protruding tab portions 469 formed on a pair of opposed edgesof the through opening 468. The tab portions 469 are dimensioned to abutflanges 312 formed on a tubular form 304 (shown in FIGS. 16 and 17) ofthe RF coil assembly 300, as discussed further below.

Referring to FIG. 6, the spacer assembly 400 also includes a pair ofelectrically conductive, non-magnetic shield plates 490, 496, which aretypically formed of copper. The shield plates are rectangular in shapeto conform to the shapes of the spacer body 402 and the free ends of themagnet units 260, 280 (e.g., the pole face second sides 270, 290), butare not limited to this shape. The shield plates 490, 496 are thin(e.g., the height or thickness is less than the length and width), whilethe thickness of the shield plates is equal to or greater than theminimum skin depth for the working frequency of the RF coil 306. In someimplementations, the skin depth is about 25 μm, and the shield plates490, 496 have a thickness in a range of 50 μm to 100 μm.

Referring to FIGS. 6 and 8, when the spacer assembly 400 is assembled,one of the shield plates 490 is disposed between the spacer body firstside 404 and the first pole piece second end 270, and another of theshield plates 496 is disposed between the spacer body second side 406and the second pole piece second end 290. The shield plates 490, 496 aretypically not mechanically secured to the spacer assembly 400. Rather,they are clamped between the magnet units 260, 280 in the desiredarrangement due to the attractive magnetic force between the magnets261, 281. In this configuration, the shield plates 490, 496 serve toshield the pole pieces 266, 286 from the RF field generated by the RFcoil assembly 300. As a result, acoustic ringing in the pole pieces 266,286 can be reduced or eliminated. Including the shield plates 490, 496in the spacer assembly 400 can also improve the Q factor (e.g., coilsensitivity) of the RF coil 306 relative to an assembly without theshield plates 490, 496.

To provide a magnetic field B0 between the first magnet unit 260 and thesecond magnet unit 280 that is sufficiently homogeneous for measurementof the concentration of a substance disposed in the magnetic field B0,the second face 270 of the first pole piece 266 is orientedsubstantially parallel to the second face 290 of the second pole piece286. For example, the faces 270, 290 can be angled by no more than 0.2degrees (e.g., 0.1 to 0.2 degrees, 0.14 degrees) relative to oneanother. In use, the spacer assembly 400, including the spacer body 402having parallel opposed first and second sides 404, 406, is placedbetween the first magnet unit 260 and the second magnet unit 280. Thespacer assembly 400 is retained between the first magnet unit 260 andthe second magnet unit 280 via the mechanical force caused by the strongmagnetic attraction between the two assemblies 260, 280 (e.g., thespacer assembly 400 is clamped between the first magnet unit 260 and thesecond magnet unit 280).

In the NMR sensor assembly 200, the support frame 201, the magnet units260, 280 and the spacer assembly 400 cooperate to provide a balancedmagnet design, as illustrated in FIG. 5. A moderate imbalance betweenthe magnetic forces in the two secondary air gaps g₁, g₂ between thefirst frame member 202 and the second frame member 232 can be tolerated.For example, the assembly will only pivot about the left edge of thespacer assembly 400 if the following equation is satisfied:

${{F_{l}\left( \frac{d - w}{2} \right)} > {F_{r}\left( \frac{d + w}{2} \right)}},{{{i.e.\mspace{11mu} {if}}\mspace{14mu} F_{l}} > {F_{r}\left( \frac{d + w}{d - w} \right)}}$

In this equation, F_(l) is the magnetic force across one of the air gapsg (e.g., the upper air gap g₁), F_(r) is the is the magnetic forceacross the other one of the air gaps g (e.g., the lower air gap g₂), dis a distance from a coil axis 316 (shown in FIG. 16-18) to the main airgap g_(m), and w is a width of the spacer assembly 400.

In addition to increased mechanical stability as compared to some otherNMR magnet assembly designs, such as certain C-core magnet assemblydesigns, the balanced magnet design has additional benefits. It canprovide a slightly stronger B0 field and less stray field than someother NMR magnet assembly designs such as certain C-core magnet assemblydesigns. Conversely, it is relatively insensitive to nearby magnets andferromagnetic parts. For example, bringing another large magnet as closeas 50 mm to the support frame 201 may only alter the magnetic field B0by 0.001 T in certain implementations.

As shown in FIGS. 13 and 15, the spacer body 402 is equipped with abarcode reader 498. The barcode reader 498 can be used to automaticallyread a barcode on the blood cartridge 500 when the blood cartridge 500is inserted into the NMR sensor assembly 200. The barcode reader 48 isconnected to the controller 220 of the NMR sensor assembly 200 such thatinformation read by the barcode reader 498 can be transmitted to thecontroller 220.

Referring to FIGS. 16-18, the RF coil assembly 300 includes the tubularform 304 having a generally rectangular cross section, and a conductivewire 302 wrapped around the form 304 to form a radio frequency (RF) coil306 having a coil axis 316. As used herein, the term “RF coil” refers toa coil configured to emit and/or detect radio frequency electromagneticenergy. When the RF coil is configured to emit and/or detectelectromagnetic energy at a particular frequency, the RF coil can besaid to be tuned to that particular frequency. In some implementations,the conductive wire 302 is an enamel-insulated solid copper wire havinga gauge of 0.5 mm that is wound 10 turns about the form 304.

Each of the opposed ends 308, 310 of the form 304 are provided with aflange 312, which has a rectangular profile. When the RF coil assembly300 is assembled with the first support plate 460 and the second supportplate 480, the RF coil assembly 300 is disposed within the throughopenings 468, 488 of the plates 460, 480 so that the flanges 312 of thecoil wire form 304 abut the tab portions 469 of the through opening 468(shown in FIG. 14) and tab portions 489 of the through opening 488(shown in FIG. 15) in a press fit relationship. In addition, when the RFcoil assembly 300 is assembled with the first and second support plates460, 480, and the first and second support plates 460, 480 are disposedwithin the respective grooves 408, 410 of the spacer body 402, the coil306 surrounds a portion of the spacer body internal space 420 and thecoil axis 316 is oriented transverse to an axis aligned with themagnetic field B0. Respective ends 302 a, 302 b of the conductive wire302 are electrically connected to the printed circuit board 350 (shownin FIG. 3) mounted on the spacer body flange 422. The printed circuitboard 350 includes an NMR sensor assembly control unit 220 and anelectronic circuit 600 used to operate the NMR sensor assembly 200, asdiscussed further below.

A mathematical representation of electromagnetic energy in the radiofrequency range is sometimes called an electromagnetic signal (or simplya signal). The RF coil 306 is used in an electronic circuit 600 (shownin FIG. 19) that detects signals caused by the transmission ofelectromagnetic energy, such as the signals emitted by sodium atoms whenthe atoms are exposed to electromagnetic energy. The detected signalcarries information about how many sodium atoms are in the blood flowingthrough the cartridge 500. Sodium atoms precess at a frequency of 11.262MHz/Tesla. In some implementations, a magnetic field of 0.8 Tesla isused. In that magnetic field, sodium atoms precess at a frequency ofabout 9.0 MHz.

The electronic circuit 600 that includes the RF coil 306 is configuredto switch between a transmitter mode and a receiver mode. For example,transistors can be used to switch between voltage sources or sinks thatare connected to the circuit. In this way, the same RF coil 306 can bothtransmit a signal and receive a signal emitted by the sodium atomscaused to precess by the transmitted signal. The transmitted signal isusually generated using a high voltage (e.g., 100 volts) so that thetransmitted signal carries enough energy to incite the sodium atoms toprecess. In contrast, the signal emitted by the sodium atoms istypically very weak, by which we mean that the signal represents a verysmall amount of energy. For example, if a signal is transmitted using avoltage of 100 volts, then the received signal may induce a voltage ofonly several microvolts.

A weak signal is susceptible to noise, which is a by-product ofelectrical energy flowing through other components of the circuit (e.g.,the transistors of the circuit) that includes the RF coil 306. Thepresence of noise in the signal reduces the ability of an NMR system touse the information carried by the signal to accurately measure theamount of sodium atoms in the blood sample of a patient.

To reduce noise, the circuit 600 includes a low noise amplifier (LNA)614, which is an electronic component that increases the energy of asignal while minimizing the amount of noise introduced as part of theamplification. In this way, the LNA 614 outputs a signal that isstronger, carries little additional noise, and is less susceptible tonoise introduced by other components of the electronic circuit. However,because the LNA 614 cannot withstand high voltages, the LNA 614 isconnected to the transistors used to switch between the transmitter modeand the receiver mode of the circuit in a way that isolates the LNA 614from the high voltages of the transmitter mode. Thus, the completecircuit 600 that incorporates the RF coil 306, the LNA 614, andtransistors introduces noise into signals detected by the RF coil 306,because the transistors have electromagnetic characteristics, such asimpedance, that affect electromagnetic signals carried by the circuit.Further, the combination of the RF coil 306 together with resonantcomponents, such as a capacitor, has a high impedance so that the coilhas high sensitivity at its resonant frequencies. For this reason, thecombination of the RF coil 306 and a capacitor is sometimes called ahigh impedance RF coil or a resonant RF coil. In the circuit 600including the high impedance RF coil 306 (e.g., an RF coil with animpedance of 10K ohms or more), the ratio of the impedance of the RFcoil 306 to the impedance of some of the other components (such as thetransistors of switching circuits) will be relatively high at resonantfrequencies of the high impedance RF coil 306. Because the LNA 614 isconnected in parallel with the high impedance RF coil 306, the impedanceof the LNA 614 will be higher still. For example, the ratio of theimpedance of the LNA 614 to the high impedance RF coil 306 may be 10:1(e.g., if the impedance of the RF coil 306 is 10K ohms and the impedanceof the LNA 614 is 100K ohms). Thus, relatively little current will flowacross the LNA 314 compared to the current that flows across the highimpedance RF coil 306, and so relatively little energy will be lost atthe LNA 314 when the circuit 300 carries a signal (such as a signaldetected by the RF coil 306).

FIG. 19 shows a block diagram of the circuit 600 used in the NMR sensorassembly 200. The circuit 600 includes an RF coil 306 that can be used,in some examples, to both excite sodium atoms and to detect energyemitted by excited sodium atoms. The RF coil 306 is connected to aresonant circuit 602. A resonant circuit is a circuit that includescapacitors and/or inductors and is arranged to oscillate at a particularfrequency. In this way, a signal carried across a resonant circuit atthe frequency of the resonant circuit is carried across the circuit witha minimal loss of energy. In this example, the components of theresonant circuit 602 are configured to resonate at a resonant frequencyof the RF coil 306 (e.g., at the precession frequency of sodium atoms).

The resonant circuit 602 connects to an isolation circuit 604. Thecircuit 600 cannot both transmit and receive at the same time, so theisolation circuit 604 includes one or more switches that isolate thetransmitting portions of the circuit 600 from the receiving portions ofthe circuit 600.

When the isolation circuit 604 is switched to a transmitting mode, atransmitting amplifier 608 is activated, which amplifies a signal forthe sodium frequency (e.g., a signal at a frequency of 11.262 MHz/Teslaof magnetic field). The signal causes the RF coil 306 to generate anelectromagnetic field that excites sodium atoms. A clock generationcircuit 612 generates signals that pass through the amplifier 608.

When the isolation circuit 604 is switched to a receiving mode, a signalreceived by the RF coil 306 passes through the resonant circuit 602 tothe low-noise amplifier 614, which amplifies a signal for the sodiumfrequency. The analog output of the low-noise amplifier 614 is providedto an analog-digital converter 618 (ADC), which converts the analogoutput to one or more digital signals. The digital signals are providedto, and analyzed by, digital components to determine information aboutthe sodium in a patient's blood. The digital components include amicrocontroller 610, memory 616, and a computer system 620. In thisexample, the computer system 620 represents the dialysis machine 100. Insome implementations, the digital components also include afield-programmable gate array 622 (FPGA). For example, the FPGA 622 canbe used to synchronize the transmitting and receiving modes for each ofthe two frequencies and implement the timing for the NMR pulse sequence.

The RF coil 306 has a high impedance (e.g., 10K ohms or more, 50K ohmsor more). When the isolation circuit 604 is switched to a transmittingmode (e.g., when the RF coil 306 transmits a signal to excite sodiumatoms), the receiving components are isolated from the high voltages ofthe transmitting mode. When the isolation circuit 604 is switched to areceiving mode (e.g., when the coil is waiting to detect electromagneticfields generated by the precession of the excited atoms), thetransmitting components are isolated from the low voltage receivingcomponents. When combined with the RF coil 306, this arrangement allowslow voltage electronic components to be placed on the circuit 600without the risk of damage.

Some transmitter/receiver circuits use a matching circuit to connect ahigh impedance coil to a low impedance (e.g., 50 ohms) load. However, inthe circuit 600, the switching components (e.g., the isolation circuit604) are directly connected to the RF coil 306 and no impedance matchingcircuit is used. If the RF coil 306 were operated at a low impedance,the coil would lose sensitivity due to energy lost from the impedance ofthe components of the isolation circuit 604. The loss of energy wouldincrease the amount of time needed to receive enough energy to analyzethe signal in the receiving mode.

Since the RF coil 306 has a high impedance, the RF coil 306 can beconnected to a high-impedance low-noise amplifier (e.g., low-noiseamplifier 614). A high-impedance low-noise amplifier is used so that amatching circuit is not required to connect the low-noise amplifier 614and the RF coil 306.

FIG. 20 shows a circuit 700 implementing a portion of the functionalityof the circuit 600 shown in FIG. 19. For example, the components of thecircuit 700 (depending on their configuration) can be used as theportion of the circuit 600 used to detect sodium atoms. The RF coil 306of FIG. 19 is shown as a current source 702, an inductor 706, and aresistor 708, which represent the current generated by the coil, theinductance of the coil, and the resistance of the coil, and theinductance of the coil, respectively. The combination of the currentsource 702, the inductor 706, and the resistor 708 is connected inparallel with a capacitor 704. In some examples, the capacitance of thecapacitor 704 can be 650 picofarads, the inductance of the inductor 706can be 1 microhenry, and the resistance of the resistor 708 can be 0.3ohms. The RF coil 306 has a high impedance when the capacitor 704 isused.

The circuit 700 switches between a transmitting mode and a receivingmode using transistors 710, 712. The first transistor 710 has a firstvoltage source 714 connected to a gate input of the first transistor710, and the second transistor 712 has a second voltage source 716connected to a gate input of the second transistor 712. The firsttransistor 710 has a source input connected to ground, and the secondtransistor 712 has a source input connected to a third voltage source718.

When the first transistor 710 is switched off and the second transistor712 is switched on, the circuit 700 is in a receiving mode (FIG. 20).The first voltage source 714 applies 0 volts to the gate input of thefirst transistor 710 to switch it off, and the second voltage source 716applies 80 volts to the gate input of the second transistor 712 toswitch it on. The third voltage source 718 is set to 100 volts, suchthat the source input of the second transistor 712 is a path to ACground across a capacitor 720. This capacitor 720 is a variablecapacitor that can be used to adjust the receiving frequency. In someexamples, the capacitor 720 has a capacitance of 315 picofarads. Currentalso flows through another capacitor 722. This capacitor 722 is avariable capacitor that can be used to adjust the transmittingfrequency. In some examples, the capacitor 722 has a capacitance of 185picofarads. Current also flows through a resistor 724 and a capacitor726 arranged in parallel, which are representations of the inputresistance and capacitance of a low noise amplifier. In some examples,the resistor 724 has a resistance of 500K ohms, and the capacitor 726has a capacitance of 2 picofarads. When the first transistor 710 isswitched on and the second transistor 712 is switched off, the circuit700 is in a transmitting mode (FIG. 21). The first voltage source 714connected to the gate of the first transistor 710 applies a voltage of20 volts. The first transistor 710 provides a path to ground across acapacitor 728. In some examples, the capacitor 728 has a capacitance of1 microfarad. The second and third voltage sources 716, 718 connected tothe second transistor 712 apply a voltage of negative 100 volts acrossthe gate and source inputs of the second transistor 712, respectively.In this way, a high voltage is applied to the circuit 700 when it isoperating in the transmitting mode.

The transistors 710, 712 each have a low on-state resistance to avoidthe loss of energy in the circuit. The transistors 710, 712 operate at ahigh voltage to withstand the high voltages used (e.g., the voltagesprovided by the second and third voltage sources 716, 718). For example,the transistors 710, 712 maintain a switching state (e.g., the on or offstate that the transistors are designed to maintain under normaloperating conditions) when a voltage of at least 100 volts is applied toany of the inputs (e.g., source, gate, or drain).

In the arrangement represented by the circuit 700, there are no largeinductive elements (e.g., coils with ferrites), so the receiverelectronics can be mounted close to the dual tuned coil 306 within themagnetic field without affecting the operation of the circuit 700.

In the NMR sensor assembly 200, the RF coil assembly 300 is disposed inthe gap g_(m) between the first magnet unit 260 and the second magnetunit 280 so that the coil axis 316 is transverse to the magnetic fieldB0.

Referring briefly again to FIG. 4, when the blood cartridge 500 isdisposed in the spacer assembly 400 of the NMR sensor assembly 200, afluid passageway of the cartridge 500 resides within the RF coil 306.The axis 316 of the coil 306 is perpendicular to the magnetic field B0.During use, blood flows through the fluid passageway of the bloodcartridge 500 and sodium atoms in the blood within the fluid passagewaybecome aligned with the magnetic field B0. Once the sodium atoms havebecome aligned with the magnetic field B0, RF energy is transmitted bythe RF coil 306 to excite the sodium atoms in the blood causing theaffected atoms to align with the transmission field (i.e., along thecoil axis 316) rather than the main field B0. Since the coil axis 316 istransverse to the magnetic field B0, the RF energy transmitted by the RFcoil 306 temporarily changes alignment of atoms to a directiontransverse to the main field B0.

NMR technology exploits the fact that the nuclei of some atoms—such ashydrogen (1H) and sodium (23Na) atoms—have a magnetic moment due totheir spin. Although the behavior of such a nucleus is governed byquantum mechanics, it can be understood in classical terms as small,spinning magnet having the following properties:

-   -   1) In the presence of a static magnetic field, its spin axis        aligns with the field (e.g., the static magnetic field B0 that        is generated in the gap gm of the NMR sensor assembly 200);    -   2) If its axis is tilted away from the magnetic field B0 (e.g.,        by transmitting an RF pulse from the RF coil 306), the axis        precesses at a frequency proportional to the strength of the        magnetic field B0; and    -   3) The precessing nucleus generates a rotating magnetic field        that induces an AC voltage in the nearby RF coil 306.

The precession frequency is determined not only by the strength of B0but also by the type of atom, quantified by its gyromagnetic constant.This is the basis for distinguishing different types of atoms in asample. For example, in a 1 Tesla field, 1H nuclei precess at afrequency of 42.0 MHz, while 23Na nuclei precess at a frequency of 11.6MHz. The RF coil 306 is tuned so as to permit transmission and receptionof signals at the frequency of sodium. The strength of RF energyreceived by the RF coil 306 is proportional to the number of sodiumatoms in the blood sample analyzed. Provided the sample volume is fixed,the RF signal is similarly proportional to the sodium concentration inthe blood sample.

The blood cartridge 500 is designed to hold a blood sample within thespacer assembly 400 in such a way that the blood flowing through thecartridge 500 resides within the magnetic field B0 for a desired lengthof time, and then is directed through the RF coil 306 where a sodiummeasurement is performed on the blood.

As shown in FIGS. 22-30, the cartridge 500 includes a rigid base 502,and a rigid cover 504 that is attached to one side of the base 502 in afluid-tight manner. In some implementations, the cover 504 is welded tothe base 502. However, other attachment techniques can alternatively oradditionally be used.

The base 502 includes a flat plate 510 having an irregular peripheralshape, and an outer sidewall 512 that extends in a direction normal tothe plate 510 along the periphery of the plate 510 so as to surround theplate 510. The base 502 includes a through opening 514 formed at alocation spaced apart from the outer sidewall 512. The base 502 alsoincludes an inner sidewall 516 that extends in a direction normal to theplate 502 along the periphery of the through opening 514 so as tosurround the through opening 514. When the cover 504 is connected to thebase 502, the cover 504, the plate 510, the outer sidewall 512 and theinner sidewall 516 cooperate to form fluid passageways 522, 524, 526through the cartridge 500, as discussed further below. The outersidewall 512 includes a first opening 530 corresponding to a fluid inletof the cartridge 500, and a second opening 532 corresponding to a fluidoutlet of the cartridge 500. Fluid line connectors 534 are provided onan outer surface of the outer sidewall 512 at locations corresponding tothe fluid inlet 530 and fluid outlet 532. The fluid line connectors 534permit connection of blood lines of the blood line set 140 to thecartridge 500.

Referring to FIG. 27, when seen in plan view, the plate 510 generallyhas a T-shape including a horizontal portion 510 a that extendsgenerally linearly between the fluid inlet 530 and the fluid outlet 532,and a vertical portion 510 b that intersects a mid-region of thehorizontal portion 510 a. As used herein, the terms vertical andhorizontal refer to the orientation illustrated in FIG. 27, and are notintended to be limiting.

When seen in plan view, the outer sidewall 512 has an irregular shapecorresponding to the shape of the base 502. In particular, the outersidewall 512 includes a first portion 540 having a U-shape including afirst side 540 a, a second side 540 b, and a closed end 540 c joiningthe first side 540 a and the second side 540 b. The outer sidewall 512includes a second portion 550 having a U-shape including a first side550 a, a second side 550 b, and a closed end 550 c joining the firstside 550 a and the second side 550 b. The second side 550 b of thesecond portion 550 is parallel and adjacent to the first side 540 a ofthe first portion 540, and is connected to the first side 540 a of thefirst portion 540 by a connecting portion 545. As a result, a gap 536 isformed between the first portion 540 and the second portion 550. Thefirst portion 540 is shorter than the second portion 550. That is, thefirst portion closed end 540 c is closer to the through opening 514 thanthe second portion closed end 550 c. The first portion 540 is wider thanthe second portion 550. In other words, the first portion sides 540 a,540 b are further apart than the second portion sides 550 a, 550 b. Thefirst portion 540, the second portion 550 and the gap 536 correspond tothe vertical portion 510 b of the T-shaped plate 510. The outer sidewall512 also includes a generally linear third portion 560. The thirdportion 560 extends in a direction transverse to the respective firstand second sides 540 a, 550 a, 540 b, 550 b of the first and secondportions 540, 550. The third portion 560 corresponds to the horizontalportion 510 a of the T-shaped plate.

Like the outer sidewall 512, when seen in plan view, the inner sidewall516 also has an irregular shape. The inner sidewall 516 is surrounded byand spaced apart from the outer sidewall 512. In particular, the innersidewall 516 includes a directing portion 580 having a U shape includinga first side 580 a, a second side 580 b, and a closed end 580 c joiningthe first side 580 a and the second side 580 b. The directing portionfirst side 580 a extends into the outer sidewall second portion 550 soas to form the alignment meandering passageway 522 within the secondportion 550. The directing portion second side 580 b extends into theouter sidewall first portion 550 so as to form the perturbationmeandering passageway 524 within the first portion 540. The directingportion closed end 580 c is generally parallel to and spaced apart fromthe outer sidewall connecting portion 545. The direction portion closedend 580 c and the outer sidewall connecting portion 545 cooperate topermit fluid communication between the alignment meandering passageway522 and the perturbation meandering passageway 524.

Still referring to FIG. 27, the inner sidewall 516 also includes a baseportion 570 that extends parallel to the outer sidewall third portion560. The space between the inner sidewall base portion 570 and the outersidewall third portion 560 defines a bypass fluid passageway 526 thatdirects fluid in a generally linear path between the cartridge inlet 530and the cartridge outlet 532. The base portion 570 is longer than awidth of the vertical portion 510 b. One end of the base portion 570cooperates with the outer sidewall 512 to define a meander inlet 522 ato the alignment meandering passageway 522 that is in fluidcommunication with, and oriented in parallel to, the bypass passageway526 at a location adjacent to the cartridge inlet 530. An opposed end ofthe base portion 570 cooperates with the outer sidewall 512 to define ameander outlet 524 a from the perturbation meandering passageway 524that is in fluid communication with, and oriented in parallel to, thebypass passageway 526 at a location adjacent to the cartridge outlet532.

The cartridge 500 is connected to the blood line set 140 (shown in FIG.1), which forms the blood circuit 10 of the hemodialysis system 101. Asa result, blood flowing through the blood line set 140 passes throughthe cartridge 500. Still referring to FIG. 27, a portion (e.g., 15 to 25percent, 20 percent) of the blood that enters the cartridge inlet 530 isdiverted into the inlet 522 a of the alignment meandering passageway522, while the remainder of the blood is directed through the bypasspassageway 526, and exits the cartridge 500 via the outlet 532. To thisend, the cross sectional area A1 of the meander inlet 522 a is smallerthan the cross sectional area A2 of the bypass passageway 526. Forexample, the cross sectional area A1 of the meander inlet 522 a can bein a range of 15 to 25 percent (e.g., 20 percent) of the cross sectionalarea A2 of the bypass passageway 526. In some implementations, the crosssectional area A2 of the bypass passageway 526 is at least 5 timesgreater (e.g., 5-15 times greater) than the cross sectional area A1 ofthe meander inlet 522 a.

Referring to FIGS. 4, 5, 27, 29 and 30, when the cartridge 500 isinserted into the spacer assembly 400, the cartridge vertical portion510 b is inserted into the spacer body opening 430 so that the cartridgevertical portion 510 b resides within the spacer internal space 420, andthus within the main air gap g_(m) of the NMR sensor assembly 200 (FIG.5). In particular, the cartridge outer sidewall first portion 540 isdisposed inside the RF coil 306 such that the first and second sides 540a, 540 b of the cartridge outer sidewall first portion 540 and secondside 580 b of the inner sidewall first portion 580 are parallel with thecoil axis 316. In addition, the cartridge outer sidewall second portion550 is disposed outside the RF coil 306 such that the first and secondsides 550 a, 550 b of the cartridge outer sidewall second portion 550and the first side 580 a of the inner sidewall first portion 580 areparallel with the coil axis 316, and so that a portion of the RF coil306 resides within the gap 536 between the outer sidewall first portion540 and the outer sidewall second portion 550.

The cartridge horizontal portion 510 a remains outside the spacer body402 and extends parallel to the spacer body first edge 412. By thisarrangement, the bypass passageway 526 remains outside both the spacerbody 402 and the main air gap g_(m) of the NMR sensor assembly 200.

The alignment and perturbation meandering passageways 522, 524 areconfigured to slow down the rate of fluid flow therein relative to therate of flow through the blood lines connected to the cartridge 500 andthe rate of flow through the cartridge inlet 530. This is accomplishedby providing the alignment meandering passageway 522 with a reducedcross sectional area relative to the cross sectional area A3 of thecartridge inlet 530 and the cross sectional area A2 of the bypasspassageway 526, as shown in FIGS. 22-26, and by including multiplechanges in direction of the fluid flow. The cross sectional area A4 ofthe alignment meandering passageway 522 is equal to the cross sectionalarea A1 of the meander inlet 522 a, and the cross sectional area A5 ofthe perturbation meandering passageway 524 is greater than that of thecross sectional area A4 of the alignment meandering passageway 522 andthe cross sectional area A1 of the alignment meandering passageway 522.For example, the cross sectional area A5 of the perturbation meanderingpassageway 524 can be in a range of 2 to 10 times greater than the crosssectional area A4 of the alignment meandering passageway 522 and thecross sectional area A1 of the alignment meandering passageway 522. As aresult, the rate of fluid flow within the alignment meanderingpassageway 522 is less than that of the bypass passageway 526, and therate of fluid flow within the perturbation meandering passageway 524 isless than that of the alignment meandering passageway 522. In someimplementations, the blood flows through the bypass passageway 526 at arate of 400 ml/min to 600 ml/min (e.g., 500 ml/min), the blood flowsthrough the alignment meandering passageway 522 at a rate of 50 ml/minto 200 ml/min (e.g., 125 ml/min), and the blood flows through theperturbation meandering passageway 524 at a rate of 50 ml/min to 150ml/min (e.g., 100 ml/min). Due to the differing lengths and flow areasof the passageways 522, 524, 526, during a hemodialysis treatment, ittypically takes 150 msec to 300 msec (e.g., 200 msec) for the blood toflow through each of the alignment meandering passageway 522 and theperturbation meandering passageway 524. It typically takes significantlyless time for the blood to flow through the bypass passageway 526.

Sharp corners within the fluid passageways 520 are avoided to preventblood stagnation that can lead to coagulation. In addition, the cassette500 may include other features that help to prevent shearing in the flowand thus help to ensure that the blood passing through the cassette 500is not damaged. For example, the inlet geometry can be configured toavoid the formation of a jet by ensuring a smooth transition between theinlet 534 and the region immediately inside the cassette 510. Similarly,a smooth transition can be provided between region 510 and where theflow subsequently divides between the meander passageways 522 a and thebypass passageway 526.

Knowing the precise volume of the blood sample to be analyzed canfacilitate accurate determination of the concentration of sodium in theblood flowing through the cartridge 500. Referring to FIGS. 28-30, tothis end, during or after manufacture of the cartridge 500, the volumeV1 of the portion of the perturbation meandering passageway 524 thatresides within the RF coil 306 is measured, and a side of the cartridge500 is marked to include a barcode 506 to indicate its volume. Any ofvarious techniques that are capable of precisely determining the volumeV1 of the portion of the perturbation meandering passageway 524 thatresides within the RF coil 306 can be used. In some implementations, acontact probe (e.g., the Equator 300 contact probe manufactured byRenishaw) can be used to measure the volume V1. A single spot laser(e.g., the M7L/50 single spot laser manufactured by MEL) canalternatively or additionally be used to measure the volume V1. Asdiscussed above, the barcode scanner 498 of the spacer assembly 400 canautomatically read the barcode 506 of the cartridge 500 when thecartridge 500 is inserted into the spacer body 402, and can transmitthat information to the NMR sensor assembly controller 220.

FIG. 31 illustrates a reference fluid cartridge 2000 that can be used tohold a reference fluid sample within the spacer assembly 400 duringcalibration of the NMR sensor assembly 200. The reference fluidcartridge 2000 includes a rigid base 2002, and a rigid cover 2004 thatconnects to one side of the base 2002 in a fluid-tight manner. In someimplementations, the cover 2004 is welded to the base 2002. However,other attachment techniques can alternatively or additionally be used.

The base 2002 includes a flat plate 2010 having a generally rectangularperipheral shape, and an outer sidewall 2012 that extends in a directionnormal to the plate 2010 along the periphery of the plate 510 so as tosurround the plate 2010. The base 2002 also includes an inner sidewall2016 that extends in a direction normal to the plate 2010. When thecover 2004 is connected to the base 2002, the cover 2004, the plate2010, the outer sidewall 2012 and the inner sidewall 2016 cooperate toform a fluid reservoir 2522 within the cartridge 2000, which has thesame size and shape as the portion of the perturbation meanderingpassageway 524 of the cartridge 500 that resides within the RF coil 306.The reservoir 2522 is filled with a solution including a knownconcentration of sodium. Like the cartridge 500, the reference fluidcartridge 2000 is marked during or after manufacture with a barcode2506, in this case indicating the concentration of the sodium within thereservoir 2522. When the reference fluid cartridge 2000 is inserted intothe spacer body 402 of the NMR sensor assembly 200, the barcode reader498 reads the barcode 2506, and transmits the read information to theNMR sensor assembly controller 220. A difference between the actualsodium concentration of the reference fluid indicated by the barcode2506 and the concentration of the reference fluid determined by the NMRsensor assembly 200 can be used to calibrate the NMR sensor assembly 200to ensure that accurate readings of blood sodium can be achieved whenthe NMR sensor assembly 200 is later used to measure the sodiumconcentration of a blood sample in the blood cartridge 500.

Referring to FIG. 32, the dialysis system 101 includes the blood circuit10 defined by the blood line set 140 and a dialysate circuit 12. Duringdialysis treatment, the hemodialysis machine 100 controls and monitorsthe flow of dialysate and blood through the dialysate circuit 12 and theextracorporeal blood circuit 10, respectively.

Referring particularly to the right side of FIG. 32, the dialysatecomponents of the dialysate circuit 12 that are located inside thehousing of the hemodialysis machine 100 include a first dialysate pump1204, a balancing device 1206, a pressure sensor 1208, an equalizingchamber 1210, a second dialysate pump 1212, and an ultrafiltration pump1214. These dialysate components are fluidly connected to one anothervia a series of dialysate lines 1216.

The first dialysate pump 1204 is capable of pumping fresh dialysate to achamber half 1220 of the balancing chamber 1206 via a dialysate supplyline 1126 that is connected to a dialysate source 1124, and the seconddialysate pump 1212 can be used to pump spent dialysate to a chamberhalf 1218 of the balancing chamber 1206 via a dialysate supply line 1126that is connected to the equalizing chamber 1210. In someimplementations, the dialysate pumps 1204, 1212 are peristaltic pumps.However, other types of pumps can alternatively or additionally be used.Examples of other suitable types of pumps include diaphragm pumps andgear pumps.

The balancing device 1206 includes a spherical chamber that is dividedinto the first chamber half 1218 and the second chamber half 1220 by aflexible membrane 1222. As fluid flows into the first chamber half 1218,fluid is forced out of the second chamber half 1220, and vice versa.This balancing device construction helps to ensure that the volume offluid entering the balancing device 1206 is equal to the volume of fluidexiting the balancing device 1206. This helps to ensure that the volumeof fresh dialysate entering the dialysate circuit is equal to the volumeof spent dialysate exiting the dialysate circuit when desired duringtreatment, as described in greater detail below.

An ultrafiltration line 1129 is connected to an outlet of the equalizingchamber 1210. The ultrafiltration pump 1214 is operatively connected tothe ultrafiltration line 1129 such that when the ultrafiltration pump1214 is operated, spent dialysate can be pulled from the equalizingchamber 1210 and directed to the drain via the ultrafiltration line1129. Operation of the ultrafiltration pump 1214 while simultaneouslyoperating the dialysate pump 1212 causes increased vacuum pressurewithin the dialysate line 1216 connecting the equalizing chamber 1210 tothe dialyzer 1110, and thus creates increased vacuum pressure within thedialyzer 1110. As a result of this increased vacuum pressure, additionalfluid is pulled from the blood circuit 10 into the dialysate circuit 12across the semi-permeable structure (e.g., semi-permeable membrane orsemi-permeable microtubes) of the dialyzer 1110. In certainimplementations, the ultrafiltration pump 1214 is a peristaltic pump.However, any various other types of pumps can alternatively oradditionally be used. Examples of other suitable types of pumps includediaphragm pumps and gear pumps.

A pressure sensor 1208 is also positioned along the dialysate line 1216leading from the dialyzer 1110 to the equalizing chamber 1210 formonitoring fluid pressure within the dialysate circuit 12.

FIG. 41 is a flow chart illustrating a method of determining aconcentration of a substance in a medical fluid. As shown, the methodincludes using a medical fluid pump to deliver medical fluid to a firstportion of a cartridge while the first portion of the cartridge ispositioned within a magnetic field (Step 3002), exciting atoms in themedical fluid in the first portion of the cartridge by applying radiofrequency energy to the medical fluid in the first portion of thecartridge (Step 3004), receiving radio frequency energy generated by theexcited atoms in the medical fluid in the first portion of the cartridge(Step 3006), and determining a concentration of a substance in themedical fluid based on the received radio frequency energy (Step 3008).

FIG. 42 is a flow chart illustrating another method of determining aconcentration of a substance in a medical fluid. The method includesreading an indicia of a medical fluid cartridge to determine a volume ofa fluid passageway of the medical fluid cartridge indicated by theindicia (Step 3020), receiving radio frequency energy generated byexcited atoms in medical fluid in the fluid passageway of the medicalfluid cartridge (Step 3022), and determining a concentration of asubstance in the medical fluid based on the determined volume of thefluid passageway and the received radio frequency energy (Step 3024).

FIG. 43 is a flow chart illustrating a method of measuring and marking amedical fluid cassette. The method includes determining a volume of afluid passageway of a medical fluid cartridge (Step 3032) and applyingan indicia to the cartridge where the indicia indicates the determinedvolume of the cartridge and the indicia is machine readable (Step 3034).

A method of performing hemodialysis, which includes measuring aconcentration of a sodium in a blood sample during dialysis treatmentusing the NMR sensor assembly 200 and the blood cartridge 500, will nowbe described.

Before beginning the dialysis treatment, the NMR sensor assembly 200 iscalibrated. In particular, the reference fluid cartridge 2000 includinga sample liquid (e.g., a saline solution) of known sodium concentrationis inserted into the NMR sensor assembly 200 so as to reside within theRF coil 306. The barcode reader 498 of the spacer body 402 is used toread the sodium concentration of the sample liquid (i.e., the actualreference sodium concentration) and the volume Vr of the referencecartridge reservoir 2522 from the barcode 2506 on the side of thereference fluid cartridge 2000. The information read by the barcodereader 498 is transmitted to the controller 220 and stored in memory.

The reference fluid cartridge 2000 is allowed to sit within the RF coil306 for a predetermined waiting period (e.g., 150 msec to 300 msec, 20msec) before a sodium measurement is performed. The waiting periodpermits the sodium atoms within the reservoir 2522 to become alignedwith the magnetic field B0.

After the waiting period has elapsed, the NMR sensor assembly 200 isused to measure the amount of sodium in the reference fluid contained inthe reference fluid cartridge 2000 (i.e. the measured reference sodiumquantity). This measurement of the amount of sodium in a knownconcentration permits the controller 220 to account for variations inthe NMR sensor assembly 200 from use to use, which may result, forexample, from slight changes in the uniformity of the magnetic field,trace amounts of sodium in the system, etc. The controller 220 uses thedifference between the known sodium concentration (or quantity) of thereference fluid within the reference fluid cartridge 2000 and themeasured sodium concentration (or quantity) of the reference fluid tocalibrate the NMR sensor assembly 200. For example, a machine correctionfactor CF can be calculated as follows:

CF=X _(Na actual) /X _(Na measured) =C _(Na actual) /C _(Na measured)

where

X_(Na actual) is the actual reference sodium quantity;

X_(Na measured) is the measured reference sodium quantity;

C_(Na actual) is the actual reference sodium concentration; and

C_(Na measured) is the measured/determined reference sodiumconcentration.

The machine correction factor CF is used by the controller 220 as amultiplication factor to account for variations in sensor output duringcalculations of sodium concentration. If, for example, the known actualsodium concentration of the reference fluid within the reference fluidcartridge 2000 is 125 mEq/L and the measured/determined sodiumconcentration of the reference fluid is 100 mEq/L, then subsequentsodium measurements made by the NMR sensor assembly 200 would bemultiplied by a correction factor CF of 1.25 ((125 mEq/L)/(100 mEq/L))to obtain accurate readings or determinations of those sodiumconcentrations.

After calibrating the NMR sensor assembly 200 (or the controller 220 ofthe NMR sensor assembly 200), the reference fluid cartridge 2000 isremoved from the NMR sensor assembly 200, and the blood cartridge 500,while connected in series within the blood line 1170 of the blood lineset 140 that forms the blood circuit 10 (FIG. 32), is inserted into theNMR sensor assembly 200 (FIGS. 4, 29 and 30). In particular, theperturbing meandering fluid passageway 524 is disposed inside the RFcoil 306 such that the first and second sides 540 a, 540 b of thecartridge outer sidewall first portion 540 and second side 580 b of theinner sidewall first portion 580 are parallel with the coil axis 316.The alignment meandering passageway 522 is disposed within the magneticfield B0 and outside the RF coil 306 such that the first and secondsides 550 a, 550 b of the cartridge outer sidewall second portion 550and the first side 580 a of the inner sidewall first portion 580 areparallel with the coil axis 316. The bypass passageway 526 remainsoutside the spacer body 402 and extends in parallel with the spacer bodyfirst edge 412. By this arrangement, the bypass passageway 526 residesoutside both the spacer body 402 and the main air gap g_(m) of the NMRsensor assembly 200.

With the blood cartridge 500 within the NMR sensor assembly 200, thebarcode reader 498 of the spacer assembly 400 is used to read thebarcode 506, which indicates the volume V1 of the portion of theperturbing meandering fluid passageway 524 that is disposed inside theRF coil 306 (FIG. 28), and the volume V1 is transmitted to thecontroller 220. The controller 220 is programmed to use the volume V1 incombination with a measurement of the number of sodium atoms in theblood contained in the volume V1 of the cartridge 500 to determine theconcentration of sodium in the blood. The controller 220 can, forexample, be programmed to determine the concentration of blood samplesin the cartridge 500 by dividing the measured quantity of sodium atomsX_(Na measured) by the volume V1.

Following calibration of the NMR sensor assembly 200 and determinationof the volume of the V1 of the cartridge 500, hemodialysis treatment isperformed using the hemodialysis machine 100 with the blood cartridge500 disposed within the NMR sensor assembly 200. Referring to FIG. 32,to carry out the hemodialysis treatment, the blood pump 1132 is operatedto draw the patient's blood into the blood circuit 10 via the patientline 1106. Through continued operation of the blood pump 1132, the bloodis delivered through a blood passage of the dialyzer 1110. At the sametime, the dialysate pumps 1204, 1212 are operated to draw dialysate intothe dialysate circuit 12 from the dialysate source 1124 and to cause thedialysate to flow through a dialysate passage of the dialyzer 1110. Asthe blood flows through the blood passage of the dialyzer 1110 and thedialysate flows through a dialysate passage of the dialyzer 1110,impurities and toxins are drawn from the blood into the dialysate acrossa semi-permeable structure (e.g., semi-permeable microtubes) of thedialyzer 1110. The filtered blood then flows through the blood cartridge500, air release device 1112, and patient line 1108 to be returned tothe patient. The spent dialysate (i.e., the dialysate containing theimpurities and toxins removed from the blood) is pumped to a drain viathe drain line 1128 and, in certain cases, the ultrafiltration line1129.

During the dialysis treatment, the sodium concentration of the blood inthe blood circuit 10 is measured using the NMR sensor assembly 200 andthe cartridge 500. The blood sodium concentration measurement ormeasurements can be used to ensure that the blood sodium concentrationis maintained within a desired concentration range during the treatment.In some implementations, the sodium concentration is measured once, forexample, at the beginning of dialysis treatment. In otherimplementations, the sodium concentration is measured several timesduring the treatment, including, but not limited to, a measurement atthe beginning of the dialysis treatment, a measurement midway throughthe dialysis treatment, and a measurement at the end of the dialysistreatment.

Referring to FIGS. 27 and 32, to measure the sodium concentration ofblood in the blood circuit 10, the blood pump 1132 positioned along theblood circuit 10 is used to deliver blood to the cartridge 500 in themanner described above. The blood enters the inlet 530 of the cartridge500 and a fraction (e.g., 10 percent to 30 percent, 20 percent) of theblood flow entering the blood cartridge 500 enters the alignmentmeandering passageway 522, while the remainder of the blood enters thebypass passageway 526. The blood cartridge 500 is configured so that theblood remains within the alignment meandering fluid passageway 522 for adesired period of time (e.g., at least 150 msec, 150 msec to 300 msec,200 msec) by controlling the blood flow rate within the alignmentmeandering fluid passageway 522 and the overall length of the alignmentmeandering fluid passageway 522. To that end, while in the alignmentmeandering passageway 522, the blood flow rate is typically reduced toabout 200 ml/min as compared to about 500-550 ml/min in the blood lines1170 leading to and from the blood cartridge 500. This blood residencetime within the alignment meandering fluid passageway 522 ensures thatsubstantially all the sodium atoms within the blood are aligned with themagnetic field B0.

As the blood exits the alignment meandering passageway 522, it entersthe perturbation meandering passageway 524. The blood cartridge 500 isconfigured so that the blood remains within the perturbation meanderingfluid passageway 524 for a desired period of time (e.g., at least 150msec, 150 msec to 300 msec, 200 msec) by controlling the blood flow ratewithin the perturbation meandering fluid passageway 524 and the overalllength of the perturbation meandering fluid passageway 524. To that end,while in the perturbation meandering passageway 524, the blood flow rateis reduced to about 100 ml/min as compared to about 500-550 ml/min inthe blood lines 1170 leading to and from the blood cartridge 500. Thisblood residence time within the perturbation meandering fluid passageway524 ensures that sufficient RF signal is obtained to perform an accuratesodium measurement using the NMR sensor assembly 200.

The blood flow rates through the alignment meandering fluid passageway522, the perturbation meandering fluid passageway 524, and the bypasspassageway 526 are controlled by the respective flow areas of thosepassageways 522, 524, 526. Because the flow area of the bypasspassageway 526 is greater than the flow area of the alignment meanderingfluid passageway 522, the blood flows through the bypass passageway 526at a greater flow rate than the blood flows through the alignmentmeandering fluid passageway 522. As the blood exits the alignmentmeandering fluid passageway 522 and enters the perturbation meanderingfluid passageway 524, the increased flow area of the perturbationmeandering fluid passageway 524 causes the blood flow rate to decrease,while the volumetric flow rates through those passageways 522, 524 areequal.

While the blood is in the perturbation meandering passageway 524, asodium measurement is performed on the blood by the NMR sensor assembly200. During the measurement, the control unit 220 controls the RF coilassembly 300 including the RF coil 306 to transmit RF energy to, andreceive RF energy from, the blood disposed within the perturbationmeandering passageway 524.

In particular, the RF coil 306 is switched between a transmit mode and areceive mode many times to perform a scan. In the transmit mode, the RFcoil 306 transmits an RF signal pulse having a voltage of about 100 Vand a duration of about 10 microseconds to excite the sodium atoms to bemeasured in the volume V1 of the cartridge 500, causing them to precessrelative to the magnetic field B0. In the receive mode, the RF coil 306“listens” to or receives the voltage (e.g., a signal of about 100 nV)generated by the precession of the excited atoms for a duration of about10 microseconds. For example, the sequence of transmissions andreceptions performed during a scan can be a Carr-Purcell-Meiboom-Gill(CPMG) sequence in which 100 to 1500 pulses are applied to the sample,and reception is performed after each pulse. In the illustratedimplementation, the scan sequence includes about 200 pulses. Due to thetime required for the transmissions, the receptions, and the RF coil totransition between transmission mode and a reception mode and viceversa, about 200 msec is typically required to perform the 200 pulsescan sequence. A voltage signal is received following each transmissionpulse, and the voltage signals received during a scan sequence areprocessed by the controller 220 to obtain a scan voltage representingthe quantity of sodium in the sample volume. During the sodiummeasurement, about 1500 scans are performed over about a five minutemeasurement period, and the scan voltages obtained are then averaged bythe controller 220 to address scan signal noise.

The concentration of the sodium in the blood is determined based on thereceived radio frequency energy generated by the excited atoms in theblood in the perturbation meandering passageway 524 of the cartridge500, i.e., the averaged scan voltage. The averaged scan voltage ismultiplied by the correction factor CF determined during calibration ofthe NMR sensor assembly 200 to arrive at a number corresponding to thenumber of sodium atoms in the sample. With knowledge of the precisevolume V1 of the blood cartridge 500, as read from barcode 506 on theside of the blood cartridge 500, the sodium concentration is thencalculated.

As discussed in detail above, the blood cartridge 500 is configured tosupport a blood sample within the RF coil 306 such that RF signalsproportional to the amount of sodium in the blood can be obtained fromblood that flows at a high flow rate (e.g., 500-550 ml/min) through theblood lines 1170 leading to and from the cartridge 500. This can beaccomplished at least in part by providing the cartridge 500 with thebypass passageway 526, which takes the majority of the blood flowwithout a significant reduction in flow rate, while allowing theremainder of the flow to pass through the meandering passageways 522,524 within the NMR sensor assembly 200 at a reduced flow rate. Inaddition to allowing the NMR sensor assembly 200 to determine thequantity of sodium atoms in the blood sample, the lengthened meanderingpassageways 522, 524 and the slowed rate of the blood through thosepassageways 522, 524 in combination with the configuration of the NMRsensor assembly 200 to simply determine the quantity of sodium atoms inthe blood allows the NMR sensor assembly 200 to be produced relativelyinexpensively because relatively small magnets can be used in the NMRsensor assembly 200.

Referring to FIG. 32, once a patient's sodium concentration isdetermined using the NMR sensor assembly 200, that information can beused to control the patient's blood sodium. For example, the amount ofsodium in the dialysate can be adjusted during dialysis treatment tomatch the patient's initial blood sodium level. The amount of sodium inthe dialysate can, for example, be adjusted by controlling the amount ofwater that is mixed with a dialysate concentrate or vice versa.Alternatively, sodium (e.g., sodium chloride solution) or diluent (e.g.,purified water) can be added to the dialysate source 1124, as needed.

While certain implementations have been described above, otherimplementations are possible.

While the blood pump 1132 has been described as a peristaltic pump,other types of pumps can alternatively or additionally be used. Examplesof other suitable types of pumps include diaphragm pumps and gear pumps.

While the heparin pump module 134 has been described as being used toinject heparin into the blood circuit of the hemodialysis system 101, itshould be understood that any of various other drugs or supplementscould alternatively or additionally be injected into the blood circuitusing the pump module 134. It should be appreciated that inimplementations including an airless blood circuit, heparin may not benecessary.

While the NMR sensor assembly 200 has been described as being positionedalong the blood line set 140 between the dialyzer 1110 and the airrelease device 1112, the NMR sensor assembly 200 could be positioned atother locations along the blood line set 140.

While the NMR module 138 has been described and illustrated as beingpositioned on the far right side of the hemodialysis machine 101, theNMR module 138 could alternatively be positioned at a different locationwithin the module compartment of the hemodialysis machine 100. Forexample, to better balance the hemodialysis machine 101, the NMR module138 could be arranged closer to the center of the hemodialysis machine100 if the NMR module 138 is heavier than other modules in thehemodialysis machine 101.

While the NMR module 138 has been described as including a cover plate139 that helps to prevent damage to the NMR sensor assembly 200 disposedwithin the housing of the NMR module 138, in certain implementations,the NMR module includes no such cover plate.

While the NMR sensor assembly 200 has been described as being part ofthe removable NMR module 138, the NMR sensor assembly 200 couldalternatively be a permanent, fixed component of the hemodialysismachine 101.

While the magnet units 260, 280 have been described as being secured tothe support frame 201 via magnetic attraction, the magnet units 260, 280can alternatively or additionally be secured to the support frame 201using other techniques, such as mechanical fastening, chemical bonding,or welding.

While the magnets 261, 281 have been described as being formed of analloy of Neodymium, Iron and Boron, in certain implementations, they areformed of one or more other materials. Examples of other suitablematerials from which the magnets 261, 281 could be formed include alloysof Samarian and Cobalt and alloys of Aluminum, Nickel and Cobalt(“Alnico”).

In addition, while the magnets 261, 281 have been described as permanentmagnets, electromagnets could alternatively or additionally be used.

While the NMR sensor assembly 200 has been described as includingrectangular magnets 261, 281, magnets of other shapes can be used. Forexample, in some implementations, the magnets have a cylindrical shapeand are used with pole pieces 266, 286 having the shape of a truncatedcone.

While the pole pieces 266, 286 have been described as being formed ofsoft iron, they can alternatively be formed of one or more othermagnetic metals that have high magnetic permeability and/or a highsaturation level. Examples of such materials include 430FR stainlesssteel and wrought iron-cobalt alloys such as Hiperco® 50 (available fromCarpenter Products).

While the spacer body 402 of the spacer assembly 400 has been describedas being formed of a ceramic material, in certain implementations, othermaterials are used. In certain implementations, for example, the spacerbody 402 is formed of one or more electrically conductive materials,such as aluminum. In such implementations, relatively thick shieldplates could be used to reduce acoustic ringing, which results from eddycurrents.

While the RF coil has been described as being configured to generate amagnetic field along one axis, other types of RF coils can be used. Asan example, a cage coil that generates a rotational magnetic field canbe used.

While the barcode reader 498 has been described as being positioned onthe spacer body 402 of the spacer assembly 400, the barcode reader 498can be located at any location that permits it to read a barcode on thecartridge 500. Similarly, while the barcode 506 has been described asbeing provided on a particular region of the cartridge 500, it should beunderstood that the barcode 506 could be located on any portion of thecartridge 500 that is visible to the barcode reader. Moreover, while thebarcode 506 and the barcode reader 498 have been described as beingarranged so that the barcode 506 is automatically read upon insertingthe cartridge 500 into the NMR sensor assembly 200, the barcode reader498 can alternatively be positioned such that the user needs to scan thebarcode 506 prior to inserting the cartridge 500 into the NMR sensorassembly 200. The barcode reader 498 could, for example, be located nearthe display 118 of the hemodialysis machine 101.

While the NMR sensor assembly 200 has been described as including adedicated control unit 220 that controls the RF coil assembly 300 andcorresponding driving electronics and that communicates with thedialysis machine control unit (e.g., via a hard-wired or wirelessconnection), control of the NMR sensor is not limited to thisconfiguration. For example, in some implementations, the dialysismachine control unit may be configured to directly control the RF coilassembly and corresponding driving electronics.

Although the conductive wire 302 used to form the RF coil 306 has beendescribed as an enamel-insulated solid copper wire, other wireconfigurations can be used. For example, in some implementations a litzwire (e.g., thin stranded wires that are twisted or woven) could be usedto form the RF coil 306, since such wire can reduce the skin effect andproximity effect losses in conductors. While the enamel provides a verythin insulation and is thus beneficial since it minimizes the outerdimension of the RF coil 306, other insulating materials, such asplastic, could alternatively or additionally be used to coat the wire.

While the systems discussed above have been described as including RFcoils that apply RF energy to and receive RF energy from medical fluidin the medical fluid cartridge, other types of RF devices canalternatively or additionally be used. For example, a “birdcage coil,”which is a coil structure incorporating multiple capacitors so that theassembly has resonant modes that generate rotating fields, could beused. Such a coil may improve the RF coupling to the atomic nuclei,which rotate when excited.

While the first and second fluid passageways of the cartridge have beendescribed as being U-shaped, in some implementations, other types ofmeandering fluid passageways are used. Examples of other types ofmeandering fluid passageways include V-shaped passageways, W-shapedpassageways, M-shaped passageways, and any other passageways thatlengthen a flow path within a confined space.

While the fluid passageways of the cartridge have been described ashaving meandering shapes, other arrangements are possible. In certainimplementations, for example, the cartridge includes a straight fluidpassageway that passes through the NMR sensor assembly. The cartridgecan, for example, be in the form of a blood line that passes straightthrough the NMR sensor assembly. In such implementations, the NMR sensorassembly could be equipped with longer magnets and a longer RF coil toensure that the blood flowing through the straight passageway is withinthe magnet filed and RF coil for a sufficient time period to align,perturb, and analyze the sodium atoms in the blood.

While the alignment meandering fluid passageway 522, which passesthrough the magnetic field generated by the NMR sensor assembly 200 butnot through the RF coil 306, has been described as having a smaller flowarea than the perturbation meandering fluid passageway 524, whichextends through the RF coil 306, in certain implementations, the flowareas of these fluid passageways are the same. Alternatively the flowarea of the alignment meandering fluid passageway 522 can be larger thanthe flow area of the perturbation meandering fluid passageway 524 incertain implementations.

While certain techniques have been described for reducing the flow areaof the fluid passageways of the medical fluid cartridges to reduce flowrates through the fluid passageways 522, 524, other techniques forreducing the flow area of the fluid passageways 522, 524 can be used. Insome implementations, for example, columns that extend from one side ofthe fluid passageway to the other are used to reduce the flow area ofthe fluid passageway and thus reduce the flow rate of fluidtherethrough. Other features, such as baffles, can alternatively oradditionally be used in certain implementations.

While the cartridges 500, 2000 have been described as including barcodesthat provide information regarding the cartridges (e.g., a volume of thecartridge, a quantity or concentration of sodium in a fluid contained inthe cartridge, etc.), other techniques for providing such informationcan be used. In certain implementations, for example, the cartridges areequipped with radio frequency identification (RFID) tags, which can beread by an RF reader of the NMR sensor assembly. In someimplementations, the cartridges include teeth (e.g., etched or machinedteeth) that can be read by an optical reader of the NMR sensor assembly.The cartridges can alternatively include printed values that can be readby an optical reader of the NMR sensor assembly or that can be read by auser and manually entered into the hemodialysis machine 101 using thedisplay 118 and input device, such as a keyboard or touch screen.

While methods discussed above involve precisely determining the volumeof the blood cartridge and then marking the blood cartridge with anindicia that includes the volume of the blood cartridge to permit anaccurate concentration of sodium in the blood flowing through the bloodcartridge to be determined, other techniques can be used. Referring toFIG. 44, in some implementations, the calibration method includesmeasuring a quantity of a first substance in a reference fluid in areference fluid cartridge (Step 3042), measuring a quantity of a secondsubstance in the reference fluid in the reference fluid cartridge (Step3044), measuring a quantity of the first substance in a medical fluid ina medical fluid cartridge (Step 3046), measuring a quantity of thesecond substance in the medical fluid in the medical fluid cartridge(Step 3048), and determining a concentration of the second substance inthe medical fluid based on the measured quantities of the first andsecond substances in the reference fluid and the medical fluid (Step3050).

In certain implementations, the NMR sensor assembly is used to measurethe concentration or quantity of both hydrogen and sodium in a referencefluid and those measurements are used to accurately determine theconcentration or quantity of sodium in blood that is later analyzed bythe NMR sensor assembly. In such implementations, the RF coil andcircuit are configured to detect signals emitted by both sodium andhydrogen atoms. In particular, the RF coil is configured to becontrolled by electronics to transmit and receive at more than onefrequency, permitting both sodium and hydrogen atoms to be measured in agiven sample. Sodium atoms precess at a frequency of 11.262 MHz/Teslawhile hydrogen atoms precess at a frequency of 42.576 MHz/Tesla. In amagnetic field of 0.8 Tesla, sodium atoms precess at a frequency of 9.0MHz and hydrogen atoms precess at a frequency of 34.06 MHz.

Because hydrogen atoms precess at a different frequency than sodiumatoms, additional electronics functionality is used to properly measurethe voltage generated by each molecule type. In particular, the RF coilused to excite the atoms and then to detect their precession in a“listening” or receiving mode is tuned to the two different resonancefrequencies of 1H and 23Na.

FIG. 33 shows a block diagram of a version of a circuit 600 a used insuch an NMR sensor assembly. The circuit 600 a includes a dual tunedcoil 306 a that can be used to sense both sodium atoms and hydrogenatoms. As used herein the term “dual tuned RF coil” refers to an RF coilthat can be used to emit electromagnetic energy and/or detectelectromagnetic energy at two different frequencies. The dual tuned coil306 a is connected to a double tuned resonant circuit 602 a. In thisexample, the capacitors and inductors of the resonant circuit 602 areconfigured to resonate at two frequencies, each of which is a resonantfrequency of the dual tuned coil 306 a.

The resonant circuit 602 a connects to isolation circuits 604, 606. Oneisolation circuit 604 is for a sodium portion of the circuit 600, andanother isolation circuit 606 is for a hydrogen portion of the circuit600 a. The circuit 600 a cannot both transmit and receive at the sametime, so each of the isolation circuits 604, 606 includes switches thatisolate the transmitting portions of the circuit 600 a from thereceiving portions of the circuit 600 a.

When the isolation circuits 604, 606 are switched to a transmittingmode, two transmitting amplifiers 608, 610 are activated, one of whichamplifies a signal for the sodium frequency (e.g., a signal at afrequency of 11.262 MHz/Tesla of magnetic field) and one of whichamplifies a signal for the hydrogen frequency (e.g., a signal at afrequency of 42.576 MHz/Tesla of magnetic field). The signals cause thedual tuned coil 306 a to generate an electromagnetic field that excitessodium and hydrogen atoms. A clock generation circuit 612 generatessignals that pass through each amplifier 608, 610.

When the isolation circuits 604, 606 are switched to a receiving mode, asignal received by the dual tuned coil 306 a passes through the resonantcircuit 602 to two low-noise amplifiers 614, 616, one of which amplifiesa signal for the sodium frequency and one of which amplifies a signalfor the hydrogen frequency. The outputs of the low-noise amplifiers 614,616 are provided to an analog-digital converter 618 (ADC). Ananalog-digital converter takes as input an analog signal and converts itto a digital signal for use with digital components. Here, theanalog-digital converter 618 outputs digital versions of the signalsreceived and amplified by the circuit 600 a. The signals are output todigital components that analyze the signals to determine informationabout the sodium and hydrogen in a patient's blood. An FPGA 622 can beused to synchronize the transmitting and receiving modes for each of thetwo frequencies and implement the timing for the NMR pulse sequence.

The dual tuned coil 306 a is selected to have a high impedance (e.g.,10K ohms or more, 50K ohms or more). When the isolation circuits 604,606 are switched to a transmitting mode (e.g., when the dual tuned coil306 transmits a signal to excite sodium or hydrogen atoms), thereceiving components are isolated from the high voltages of thetransmitting mode. When the isolation circuits 604, 606 are switched toa receiving mode (e.g., when the coil is waiting to detectelectromagnetic fields generated by the precession of the excitedatoms), the transmitting components are isolated from the low voltagereceiving components. When combined with the dual tuned coil 306 a, thisarrangement allows low voltage electronic components to be placed on thecircuit 600 without the risk of damage.

Some transmitter/receiver circuits use a matching circuit to connect ahigh impedance coil to a low impedance (e.g., 50 ohms) load. However, inthis circuit 600 a, the switching components (e.g., the isolationcircuits 604, 606) are directly connected to the dual tuned coil 306 aand no impedance matching circuit is used. If the dual tuned coil 306 ais operated at a low impedance, the coil would lose sensitivity due toenergy lost from the impedance of the components of the isolationcircuits 604, 606. The loss of energy would increase the amount of timeneeded to receive enough energy to analyze the signal in the receivingmode.

Since the dual tuned coil 306 a has a high impedance, the RF coil 306 acan be connected to a high-impedance low-noise amplifier (e.g.,low-noise amplifiers 614, 616). High-impedance low-noise amplifiers areused so that a matching circuit need not be used to connect thelow-noise amplifiers and the dual tuned coil 306 a.

FIG. 34 shows a circuit 800 implementing a portion of the functionalityof the circuit 600 a shown in FIG. 33. One point 802 of the circuit 800is the location at which the low-noise amplifier 614 for the sodiumfrequency can be connected, and another point 806 of the circuit is thelocation at which the low-noise amplifier 616 for the hydrogen frequencycan be connected. A third point 804 of the circuit is a location atwhich a signal from the dual tuned coil 306 enters the circuit.

FIG. 35 shows a graph of the frequency response of the circuit 800 asmeasured at the first point 802. As shown in FIG. 35, the impedance hasa peak 808 at approximately 5 MHz and another peak 810 at approximately80 MHz.

FIG. 36 shows a graph of the frequency response of the circuit 800 asmeasured at the second point 2302. As shown in FIG. 36, the impedancehas peaks 812, 814 and 816 at approximately 4 MHz, 11.5 MHz, and 80 MHz.

FIG. 37 shows a graph of the frequency response of the circuit 800 asmeasured at the third point 2402. As shown in FIG. 37, the impedance haspeaks 818, 820 and 822 at approximately 5 MHz, 11.5 MHz, and 80 MHz.

FIG. 38 shows a graph of the frequency response of the dual tuned coil306 a shown in FIG. 33. As shown in FIG. 38, the magnitude of theimpedance of the coil has peaks 902, 904 at 9.5 MHz and 45 MHz, whichare approximate the frequencies of precession of sodium and hydrogenatoms in the magnetic fields used in the techniques described herein.The magnitudes of the peaks 902, 904 shown in FIG. 38 are achieved byadjusting variable capacitors in the circuit 800.

A method of using a dialysis system equipped with the dual tuned coil306 a and the electronics of FIG. 35 will now be briefly described. Itshould be understood that this dialysis system is generally the same asthe dialysis system 101 described above, except for the dual tuned coil306 a and the electronics of the NMR sensor assembly of this dialysissystem. Prior to beginning the dialysis treatment, the referencecartridge 2000, which contains a reference fluid having a knownconcentration or quantity of sodium and hydrogen, is inserted into theNMR sensor assembly and the NMR sensor assembly measures theconcentration or quantity of sodium and hydrogen in the reference fluidand stores that information in its controller. The quantity orconcentration of sodium detected by the NMR sensor assembly is thencompared to the known quantity or concentration of sodium in thereference fluid and the NMR sensor assembly is calibrated in the mannerdescribed above to account for any discrepancy between the detectedquantity or concentration of sodium and the known quantity orconcentration of sodium.

After measuring the sodium and hydrogen concentration or quantity in thereference fluid, the reference cartridge 2000 is removed from the NMRsensor assembly and the blood cartridge 500 is inserted into the NMRsensor assembly. Prior to circulating blood through the cartridge 500, asaline solution is introduced to the cartridge for priming. For example,a bag of saline solution can be connected to the blood line set and theblood pump can be operated to pump the saline solution to the bloodcartridge 500. The NMR sensor assembly then measures the quantity ofhydrogen in the saline solution and stores that value in its controller.

The hemodialysis treatment is then initiated. During the treatment bloodflows through the blood cartridge 500 and the quantity of sodium in theblood is measured by the NMR sensor assembly.

The ratio of the sodium and hydrogen signals (i.e., the signals that arereceived by the NMR sensor assembly and are indicative of the quantitiesof hydrogen and sodium, respectively, in the fluid being analyzed)depends only on the quantities or concentrations of sodium and hydrogenin the fluid, not on the cartridge volume. Both signals are proportionalto the fluid volume within the RF coil 306 a.

In addition, it is known (or can be assumed) that the hydrogenconcentration is the same in the reference fluid analyzed in thereference cartridge 2000 and the saline solution analyzed in the bloodcartridge 500. Specifically, in any dilute aqueous solution, water isthe dominant constituent and so the hydrogen concentration is very closeto that of water.

An accurate determination of the concentration of sodium in the bloodsample can be made by comparing (1) the ratio of the measured sodiumsignal in the reference fluid to the measured hydrogen signal in thereference fluid (i.e., 23Na:1H signal ratio for the reference fluid) to(2) the ratio of the measured sodium signal in the blood sample fluid tothe measured hydrogen signal in the blood sample (i.e., 23Na:1H signalratio for the blood sample). As noted above, the hydrogen concentrationin the reference fluid in the reference cartridge 2000 is equal to thehydrogen concentration in the saline solution in the blood cartridge 500since the reference fluid and the saline solution are both diluteaqueous solutions. Therefore, to the extent that the hydrogen signalsreceived during analysis of the reference fluid differ from the hydrogensignals received during analysis of the blood sample, it can be assumedthat the different readings are the result of a difference between thevolumes of the reference cartridge 2000 and the blood cartridge 500. Theratio of the sodium signal to the hydrogen signal, however, is notdependent on the volume of the sample analyzed. Therefore, if theconcentrations of sodium and hydrogen in two different samples were thesame, then the ratio of the sodium signal to the hydrogen signal wouldalso be the same, regardless of whether the sample volumes differed fromone another. In other words, even though the values or intensities ofthe sodium and hydrogen signals of a first larger volume sample maydiffer from the values or intensities of the sodium and hydrogen signalsof a second smaller volume sample, the sodium signal to hydrogen signalratio would be the same for each sample so long as the sodium andhydrogen concentrations were the same in those samples.

In view of the foregoing discussion, it will be clear that anydifference between the sodium signal to hydrogen signal ratio (23Na:1H)of the reference fluid and the sodium signal to hydrogen signal ratio(23Na:1H) of the blood sample, as measured by the NMR sensor assemblyduring the method discussed above, could be attributed to a differencebetween sodium concentrations in the reference fluid and the bloodsample. Thus, the concentration of sodium in the blood sample can bedetermined by comparing the ratio of the sodium signal to hydrogensignal ratio (23Na:1H) of the reference fluid and the sodium signal tohydrogen signal ratio (23Na:1H) of the blood sample.

As an example, suppose the ratio of the sodium signal to the hydrogensignal (23Na:1H) from the reference cartridge 2000 is 0.1 and that thereference cartridge contains a 1M sodium solution. Then, duringdialysis, a ratio of the sodium signal to the hydrogen signal (23Na:1H)of the blood sample and saline solution, respectively, in the bloodcartridge 500 is determined to be 0.01. It can be determined from thisinformation that the sodium concentration of the blood sample is 0.1 M(i.e., 0.01/0.1*1M).

A detailed explanation of the manner in which the sodium concentrationof the blood sample in the blood cartridge 500 can be determined usingthe sodium and hydrogen signals received from the blood sample and thereference fluid is provided below.

As discussed above, the ratio of hydrogen and sodium signals from asingle cartridge is independent of the machine calibration, and onlydepends on the sodium concentration.

NMR provides a signal, S, that is proportional to species concentration,C, but the gain between these depends on the gain of the machine, g,which is species dependent, and the effective volume of the cartridge,V: C=(g/V)·S

The reference cartridge 2000 holds a known volume of a knownconcentration of sodium [Na]_(ref).

The concentration of hydrogen in both the reference fluid and the salinesolution used to prime the blood line set can be assumed to be that ofwater, i.e. [H]_(H2O)=55.5M.

The sodium and hydrogen signals are both measured for the referencecartridge 2000: S_(Na,ref) and S_(H,ref). Let the effective volume ofthe reference cuvette be V_(ref). This gives two equations,

[Na]_(ref)=(g _(Na) /V _(ref))S _(Na,ref)

[H]_(H2O)=(g _(H) /V _(ref))S _(H,ref)

Taking the ratio between these determines the machine calibration:

g _(Na) /g _(H)=[Na]_(ref) S _(H,ref)/[H]_(H2O) S _(Na,ref)

Let the effective volume of the blood cartridge 500 be V_(con). Thisgives another two equations,

[Na]_(con)=(g _(Na) /V _(con))S _(Na,con)

[H]_(H2O)=(g _(H) /V _(con))S _(H,con)

Taking the ratio between these gives the following

[Na]_(con)/[H]_(H2O)(g _(Na) /g _(H))(S _(Na,con) /S _(H,con))

Using the above, the concentration of sodium in the blood can bedetermined in terms of known quantities.

While the hydrogen calibration technique described above involves theuse of the reference cartridge 2000, which contains a reference fluidhaving a known concentration or quantity of sodium and hydrogen, othertechniques can be used. In certain implementations, for example, theblood cartridge 500 is prefilled with a saline solution and is providedto the consumer in that manner. The cartridge 500 can, for example, beprovided with caps that fit over the line connectors 534 of the bloodcartridge 500 to contain the saline solution therein. Before connectingthe cartridge 500 to the remainder of the blood line set 140 in suchimplementations, the cartridge 500 would be inserted into the NMR sensorassembly 200 and used to calibrate the NMR sensor assembly 200 in themanner described above. In particular, the NMR sensor assembly 200 wouldmeasure the concentration or quantity of the sodium and hydrogen in thesaline solution and compare the measured sodium concentration orquantity to the known concentration or quantity of the sodium in thesaline solution to determine a correction factor CF to be used to adjustfuture blood sodium readings carried out by the NMR sensor assembly.After calibrating the NMR sensor assembly 200 in this manner, the bloodlines of the blood line set 140 would be connected to the bloodcartridge 500 and the hemodialysis treatment would be initiated. Thesaline solution could either be drained from the cartridge 500 prior tobeginning the treatment or simply delivered to the patient. Duringtreatment, the blood sodium concentration could be determined in themanner discussed above.

As an alternative to prefilling the cartridge 500 with saline solutionin the manner discussed above, the cartridge 500 can be provided in anempty state and the clinician can fill the cartridge 500 with salinesolution having a known sodium concentration and a known hydrogenconcentration prior to use. The calibration technique and treatment canthen be carried out in the manner described above.

In some implementations, the NMR sensor assembly is not calibrated priorto use. In some such implementations, for example, the cartridges areprecision machined to ensure that the volumes of the cartridges do notsignificantly change from one cartridge to another and to ensure thatthe actual volume of the cartridge does not significantly differ fromthe intended volume. In such cases, the cartridges wouldn't need to bemarked with their actual volume or otherwise tested to determine(directly or indirectly) their actual volume. Assuming the machine isdesigned to work with only one type of cassette, then the machine couldbe programmed (e.g., by the manufacturer) to store the intended volumeof the cassette without the need for a barcode reader or a similardevice that transmits information regarding the actual volume of thecartridge to the controller of the NMR sensor assembly. The intendedvolume could then be used in combination with the determined quantity ofsodium in a blood sample to determine the sodium concentration in apatient's blood.

In certain implementations, a relatively large cartridge is used. Such acartridge can, for example, have a volume of 5 mL to 30 mL. Due to thelarge volume of the cartridge, differences in the actual volume of thecartridge from the intended volume of the cartridge, which can resultfrom relatively imprecise manufacturing techniques, such as injectionmolding, will have a negligible affect on a sodium concentration that isdetermined by dividing a sodium quantity reading by the intended volumeof the cartridge.

While the methods discussed above involve determining actual sodiumconcentrations of the blood, in certain implementations, it is onlynecessary to monitor a change in sodium concentration over time. Thesodium concentration of the dialysate could, for example, be adjustedusing a feedback loop in response to blood sodium level changes thatoccur during treatment. As a result, it would be unnecessary to know theactual blood sodium concentration of the patient. Rather, the goal wouldbe to maintain the sodium concentration at a constant level throughoutthe treatment. In such implementations, calibration of the NMR sensorassembly and determination of the actual volume of the blood cartridgewould typically not be carried out.

While the methods discussed above involve pumping the blood through thecartridge while applying and receiving the RF energy to determine theconcentration of sodium in the blood, in some implementations, the RFenergy is applied to and received from a static sample of blood todetermine the concentration of sodium in the blood. The cartridge can,for example, include an inlet fluid passageway that leads from an inletblood line connected to the cartridge to a chamber and an outlet fluidpassageway that leads from the chamber to an outlet blood line connectedto the cartridge. The outlet blood line is connected at its opposite endto a disposable container, such as a bag or vial. The inlet and outletfluid passageways are connected to valve mechanisms that can be operatedto open and close the passageways. During use, blood is delivered to thechamber via the inlet fluid passageway with the valve mechanism closingthe outlet fluid passageway. After filling the chamber with blood, thevalve mechanism along the inlet fluid passageway is likewise closed tocontain the blood within the chamber. The NMR sensor assembly can thenbe used to determine the concentration of the static blood sample withinthe chamber. Rather than returning the blood to the patient after theanalysis, the blood is delivered to the disposable container, which canbe properly disposed of after treatment. Since the blood sample in thisvalved cartridge is static, a much smaller flow passageway would berequired in the cartridge, whereby the valved cartridge could be mademuch smaller than the previously described blood cartridges.

While the systems and methods above have been described as being used todirectly determine the concentration of sodium in a patient's blood, insome implementations, the concentration of sodium in the patient's bloodis determined indirectly, based on detected levels of sodium in thedialysate. A method of using the NMR sensor assembly 200 to indirectlymeasure blood sodium concentration using dialysate is similar to theconcept described above relating to measuring a patient's blood sodiumdirectly. However, instead of including the disposable blood cartridge500 in the blood circuit 10, diverting a portion of the patient's bloodin the disposable blood cartridge 500 into the RF coil 306, andmeasuring the flowing blood sample directly, a rigid permanent dialysatecartridge 1500 is provided in the dialysate circuit 12 having access viavalves 1502, 1504, 1506 to both the pre-dialyzer dialysate flow (e.g.,“clean” dialysate) as well as the post-dialyzer dialysate flow (e.g.,“spent” dialysate).

FIG. 39 shows schematic representations of the blood and dialysatecircuits of a hemodialysis machine configured to indirectly determinethe concentration of sodium in a patient's blood, and FIG. 40 shows adialysate cartridge 1500 that is provided along the dialysate circuitand is used to hold samples of dialysate to be analyzed by the NMRsensor assembly 200. Referring to FIGS. 39 and 40, the dialysatecartridge 1500 is in fluid communication with the dialysate circuit viaa first valve 1502 that connects a first inlet 1512 of the dialysatecartridge 1500 to the dialysate supply line 1126, a second valve 1504that connects a second inlet 1514 of the dialysate cartridge 1500 to thedialysate drain line 1128, and a third valve 1506 that connects anoutlet 1516 of the dialysate cartridge 1500 to the dialysate drain line1128 at a location between the second valve 1504 and the drain 1508. Thedialysate cartridge 1500 includes a reservoir 1518 dimensioned to bereceived within the RF coil 306 of the NMR sensor assembly 200. Thevolume of the reservoir 1518 is precisely measured at manufacture, andan outer surface of the cartridge is provided with a barcode indicatingthe volume of the reservoir 1518. The dialysate cartridge 1500 ispermanently disposed in the NMR sensor assembly 200, and is receivedwithin a modified spacer body 402′ such that the first inlet 152 and thesecond inlet 1514 extend through the spacer body opening 430, and suchthat the outlet 1516 extends through another opening 432 formed in thespacer body third edge 416. In addition, the reservoir 1518 is enclosedby the RF coil 306. Because the dialysate cartridge 1500 is typically apermanent component (i.e., not a single use component) of the dialysissystem, the dialysate cartridge 1500 and the NMR sensor assembly 200 canbe built as a permanent assembly that is housed within the main housingof the dialysis machine.

Conductivity sensors 1600 are positioned along the dialysate line 1216of the dialysate circuit upstream and downstream of the dialyzer 1110.

A method of using the NMR sensor assembly 200 to indirectly measureblood sodium concentration on the patient's dialysate obtained duringdialysis treatment will now be described.

Once hemodialysis treatment is underway such that both blood anddialysate are running through the dialyzer 1110, an on line dialysanceclearance is performed using the conductivity sensors 1600, and aclearance value (Kecn) is derived from the following formula:

Kecn=(Qd*Qf/60)*(1−((CpoUp−CpoDn)/CpiUp−CpiDn)))

-   -   where    -   Kecn is effective Na clearance    -   Qd is dialysate flow rate    -   Qf is Ultrafiltration flow rate    -   CpoUp and CpoDn are the conductivities of the dialysate post        dialyzer during the stable Up step and down steps in        conductivity    -   CpiUp and CpiDn are the conductivities of the dialysate pre        dialyzer during the stable up and down steps in conductivity.

After completion of the dialysance clearance, the dialysate conductivityis allowed to stabilize. Upon stabilization of the dialysateconductivity, the first valve 1502 and third valves 1506 are opened andthe second valve 1504 is closed while running the dialysate pumps 1204and 1212. This valve configuration permits fresh dialysate to flow fromthe fresh dialysate source 1124 through the reservoir 1518 of thedialysate cartridge 1500. Specifically, the fresh dialysate enters thecartridge 1500 via the first inlet 1512 of the cartridge 1500, travelsthrough the reservoir 1518, and then exits the cartridge 1500 via anoutlet 1516 of the cartridge 1500. The fresh dialysate is allowed toflow through the cartridge 1500 for a sufficient period of time to flushthe reservoir 1518 of any air or previously analyzed dialysate that mayhave been in the reservoir 1518. The first valve 1502 is configured suchthat fresh dialysate is also allowed to flow through the valve 1502toward the dialyzer 1110 while fresh dialysate is being delivered to thedialysate cartridge 1500. The closed second valve 1504 prevents freshdialysate from flowing through the cartridge 1500 and into the drainline 1128 via the second valve 1504 but allows spent dialysatetravelling through the drain line 1128 to pass through the second valve1504 and proceed to the drain 1508.

After the reservoir 1518 has been flushed, the third valve 1506 isclosed while the first valve 1502 remains open and the dialysate pumps1204, 1212 continue to run. This configuration directs fresh dialysateto the first inlet 1512 of the cartridge 1500, and the reservoir 1518 isfilled with fresh dialysate. Since the second and third valves 1504,1506 are closed, the fresh dialysate is not allowed to pass through thecartridge 1500. Once the reservoir 1518 is filled, the fresh dialysatefirst valve 1502 is closed so that there is no fluid flow to or throughthe cartridge 1500. However, fresh dialysate continues to pass throughthe first valve 1502 to the dialyzer 1110 and spent dialysate continuesto pass through the second valve 1504 to the drain 1508. In this way,hemodialysis treatment can resume even while the cartridge 1500 is beingfilled and the dialysate within the cartridge 1500 is being tested.

With the sample of fresh dialysate contained in the reservoir 1518, theNMR sensor assembly 200 is then operated to measure the sodiumconcentration in the fresh dialysate (CdiNa). The NMR sensor assembly200 is operated in generally the same manner as described above withrespect to measurement of sodium concentration in the blood cartridge500. Specifically, since the reservoir 1518 of the dialysate cartridge1500 is disposed within the RF coil 306, the fresh dialysate filling thereservoir resides within the magnetic field B0. With the fresh dialysatedisposed within the RF coil 306, a predetermined waiting period isallowed to elapse before a sodium measurement is performed. The waitingperiod permits the sodium atoms within the reservoir 1518 to becomealigned with the magnetic field B0. For example, the waiting period canbe in a range of 150 to 300 msec (e.g., 200 msec).

Following the waiting period, and while the fresh dialysate is disposedwithin the reservoir 1518, a sodium measurement is performed on thefresh dialysate by the NMR sensor assembly 200. During the measurement,the control unit 220 controls the RF coil assembly 300 including the RFcoil 306 to transmit RF energy to, and receive RF energy from, the freshdialysate disposed within reservoir 1518.

In particular, the RF coil 306 is switched between a transmit mode and areceive mode many times to perform a scan. In the transmit mode, the RFcoil 306 transmits an RF signal pulse having a voltage of about 100 Vand a duration of about 10 microseconds to excite the sodium atoms to bemeasured in the volume of the cartridge 1500, causing them to precessrelative to the magnetic field B0. In the receive mode, the RF coil 306“listens” to or receives the voltage (e.g., a signal of about 100 nV)generated by the precession of the excited atoms for a duration of about10 microseconds. For example, the sequence of transmissions andreceptions performed during a scan can be a Carr-Purcell-Meiboom-Gill(CPMG) sequence in which 100 to 1500 pulses are applied to the sample,and reception is performed after each pulse. In the illustratedimplementation, the scan sequence includes about 200 pulses. Due to thetime required for the transmissions, the receptions, and the RF coil totransition between transmission mode and a reception mode and viceversa, about 200 msec is typically required to perform the 200 pulsescan sequence. A voltage signal is received following each transmissionpulse, and the voltage signals received during a scan sequence areprocessed to obtain a scan voltage representing the quantity of sodiumin the sample volume. During the sodium measurement, about 1500 scansare performed over about a five minute measurement period, and the scanvoltages obtained are then averaged to address scan signal noise.

Next, the concentration of the sodium within the fresh dialysate isdetermined based on the averaged scan voltage. The average scan voltageis multiplied by the correction factor CF to arrive at a numbercorresponding to the number of sodium atoms in the fresh dialysate. Withknowledge of the precise volume of the cartridge 1500, the sodiumconcentration of the fresh dialysate (CdiNa) is then calculated.

Following the measurement of the sodium concentration in the freshdialysate, the first valve 1502 is closed, and the second and thirdvalves 1504, 1506 are opened while the dialysate pumps 1204, 1212continue to run. As a result of this valve configuration, spentdialysate is directed to the second inlet 1514 of the dialysatecartridge 1500, and the spent dialysate is allowed to flow through thedialysate cartridge 1500 for a time period sufficient to flush anyremaining fresh dialysate from the reservoir 1518. When the reservoir1518 has been flushed, the third valve 1506 is closed, and the reservoir1518 is filled with spent dialysate. Once the reservoir 1518 has beenfilled with spent dialysate, the second valve 1504 is also closed toensure that the spent dialysate sample is contained within the reservoir1518. The NMR sensor assembly 200 is then operated to measure the sodiumconcentration in the spent dialysate (CdoNa). To do this, the NMR sensorassembly 200 is operated in the same manner as described with respect tothe measurement of the fresh dialysate.

Fresh dialysate traveling through dialysate supply line 1126 is allowedto pass through the first valve 1502 to the dialyzer 1110 and spentdialysate traveling through the drain line 1128 is allowed to passthrough the second valve 1504 to the drain 1508 through the process ofthe spent dialysate being delivered to the cartridge and tested suchthat the hemodialysis treatment does not have to be stopped or pausedwhile filling the cartridge 1500 or testing the spent dialysate withinthe cartridge 1500.

Following the measurement of the sodium concentration in the spentdialysate, the controller 220 calculates the blood sodium using thefollowing formula:

Na=CdiNa(1−Qd/Kecn)(1−CdoNa/CdiNa)

-   -   where    -   CdiNa is the sodium concentration of the fresh dialysate    -   CdoNa is the sodium concentration of the spent dialysate

The underlying principle behind this method is that small molecularsolutes will pass the dialyzer's 1110 semipermeable membrane to try toreach equilibration of the solute due to diffusion gradient differences.The clearance value Kecn is used to determine the efficiency of theblood/dialysate interaction. The sodium concentration of the freshdialysate CdiNa determines the base dialysate sodium concentration, andthe sodium concentration of the spent dialysate CdoNa indicates thedirection of the equilibration of sodium. If the blood sodiumconcentration Na is higher than the base dialysate sodium concentration(e.g., higher than CdiNa), then the sodium concentration of the spentdialysate CdoNa concentration will also be higher than the base. Therate of this increase based on the clearance value Kecn allowscalculation of the concentration gradient needed in the blood to causethe increase in the sodium concentration of the spent dialysate CdoNa.The same effect occurs in the opposite direction. That is, a sodiumconcentration Na that is lower than the sodium concentration of thefresh dialysate CdiNa results in the sodium concentration of the spentdialysate CdoNa being lower than the base dialysate sodium concentration(e.g., lower than CdiNa). The rate of this decrease based on theclearance value Kecn allows calculation of the concentration gradientneeded in the blood to cause the decrease in the sodium concentration ofthe spent dialysate CdoNa.

Once the patient's initial blood sodium concentration (i.e., the bloodsodium concentration at the beginning of treatment) has been determined,the controller 220 will cause the sodium concentration of the freshdialysate to be adjusted to match the sodium concentration of thepatient's blood. For example, the amounts of water and dialysateconcentrate that are delivered to the dialysate source 1124 can beadjusted to adjust the sodium concentration of the fresh dialysate. Thiswill reduce the likelihood that the patient's blood sodium concentrationwill change during treatment. Further sodium tests can be carried duringthe treatment and the sodium concentration of the dialysate can befurther adjusted, if desired, to ensure that the patient's blood sodiumconcentration remains at or near the initial blood sodium concentrationthroughout the treatment.

The method of using the NMR sensor assembly 200 to indirectly measureblood sodium concentration using dialysate may have some advantages overthe direct blood measurement method. For example, in contrast to thedisposable blood cartridge 500, because spent dialysate is drained towaste, the permanent measurement dialysate cartridge 1500 can be reusedamong multiple patients without needing to be sterilized or replaced.Furthermore, because only one dialysate cartridge 1500 is needed perdialysis machine 100, the dialysate cartridge 1500 can be machined witha very high precision (to achieve the very precise volume required foraccurate NMR measurement), as the cost of precisely machining onepermanent cartridge per machine is much more feasible than preciselymachining millions of disposable blood cartridges 500. Finally, unlikeblood samples which must remain flowing to preserve the “non-contact”status of this technology, spent dialysate can be “pinched off” into thepermanent dialysate cartridge 1500 and measured as a static sample. Thiseliminates any concerns relating to clotting or build-up associated withuse of the blood cartridge 500, and also renders moot any effects that aflowing sample might have on NMR measurement in general.

While the NMR sensor assembly 200 of the hemodialysis system of FIGS. 39and 40 is typically not calibrated prior to use, in certainimplementations, a calibration procedure is carried out prior tobeginning dialysis treatment. In such implementations, for example,prior to initiation of dialysate flow through the dialysate circuit 12,the reservoir 1518 of the dialysate cartridge 1500 can be filled with asaline solution of known sodium concentration. This can be accomplishedby connecting a bag of saline solution having a known sodiumconcentration to the dialysate supply line 1126 and operating thedialysate pump 1204 and valves 1502, 1504, 1506 to fill the reservoir1518. After a predetermined waiting period has elapsed and allowed thesodium atoms within the saline solution in the reservoir 1518 to becomealigned with the magnetic field B0 generated by the NMR sensor assembly200, a sodium measurement is performed on the saline solution in thereservoir 1518. During the measurement, the control unit 220 controlsthe RF coil assembly 300 including the RF coil 306 to transmit RF energyto, and receive RF energy from, the saline solution disposed withinreservoir 1518. This measurement provides the measured reference sodiumquantity, which is used along with the known sodium concentration of thesaline solution and the volume of the cartridge 1500 to calculate themachine correction factor (CF). This correction factor can be applied tofuture sodium measurements to account for slight variations in theperformance of the NMR sensor assembly 200, which may occur fromtreatment to treatment or over a period of time.

While the hemodialysis system of FIGS. 39 and 40 has been described asincluding the NMR sensor assembly 200 discussed above, any of thevarious other types of NMR sensor assemblies described herein canalternatively be used in this system.

While the dialysate cartridge 1500 has been described as including a barcode that contains the volume of the dialysate cartridge 1500, in someimplementations, the dialysate cartridge includes no such bar code. Forexample, because the dialysate cartridge is precisely machined to have adesired volume, the controller of the dialysis machine can be programmed(e.g., by the manufacturer) to store the intended volume of thedialysate cartridge and that intended volume can be used to determinethe concentration of sodium in the dialysate samples contained in thedialysate cartridge.

While the systems and methods discussed above relate to determining theconcentration of sodium in a patient's blood, similar techniques can beused for determining the concentration of other substances in apatient's blood, such as calcium, phosphorous, magnesium, potassium, andother electrolytes normally found in blood.

While the systems and methods discussed above relate to determining theconcentration of a substance in the blood of a patient undergoinghemodialysis treatment, similar techniques can be used for determiningthe concentration of a substance in a patient's blood during other typesof medical treatments. Examples of such treatments includecardiopulmonary bypass procedures and plasmapheresis.

In some implementations, instructions that cause a computer to carry outone or more steps of a process are stored on a computer readable medium.Computer readable media suitable for storing computer programinstructions and data include all forms of storage devices, e.g.,non-volatile or volatile memory, media and memory devices, including byway of example semiconductor memory devices, e.g., EPROM, EEPROM, andflash memory devices; magnetic disks, e.g., internal hard disks orremovable disks or magnetic tapes; magneto optical disks; and CD-ROM andDVD-ROM disks.

Other implementations are within the scope of the following claims.

What is claimed is:
 1. A medical fluid cartridge comprising: a bodycomprising a first portion defining a first fluid passageway and asecond portion defining a second fluid passageway in fluid communicationwith the first fluid passageway, wherein the body defines a gap betweenthe first portion of the body and the second portion of the body suchthat the first portion of the body can be disposed in a radio frequencydevice while the second portion of the body remains outside the radiofrequency device.
 2. The medical fluid cartridge of claim 1, wherein thefirst fluid passageway is a meandering fluid passageway.
 3. The medicalfluid cartridge of claim 2, wherein the second fluid passageway is ameandering fluid passageway.
 4. The medical fluid cartridge of claim 1,wherein the medical fluid cartridge comprises a fluid inlet port viawhich medical fluid enters the medical fluid cartridge and a fluidoutlet port via which medical fluid exits the medical fluid cartridge.5. The medical fluid cartridge of claim 4, wherein the first fluidpassageway is configured such that fluid flows through the first fluidpassageway at a rate that is lower than a rate at which the medicalfluid flows through the fluid inlet port.
 6. The medical fluid cartridgeof claim 5, wherein the first fluid passageway is a meandering fluidpassageway.
 7. The medical fluid cartridge of claim 4, wherein thesecond fluid passageway is configured such that fluid flows through thesecond fluid passageway at a rate that is lower than a rate at which themedical fluid flows through the fluid inlet port.
 8. The medical fluidcartridge of claim 7, wherein a flow area of the second fluid passagewayis greater than a flow area of the fluid inlet port.
 9. The medicalfluid cartridge of claim 8, wherein the second fluid passageway is ameandering fluid passageway.
 10. The medical fluid cartridge of claim 1,wherein the body comprises a third portion having a third fluidpassageway.
 11. The medical fluid cartridge of claim 10, wherein thethird fluid passageway is a straight fluid passageway.
 12. The medicalfluid cartridge of claim 10, wherein the medical fluid cartridgecomprises a fluid inlet port via which medical fluid enters the medicalfluid cartridge and a fluid outlet port via which medical fluid exitsthe medical fluid cartridge.
 13. The medical fluid cartridge of claim12, wherein the third fluid passageway is configured such that fluidflows through the third fluid passageway at a rate that is lower than arate at which the medical fluid flows through the fluid inlet port andthat is greater than rates at which the medical fluid flows through thefirst and second fluid passageways.
 14. The medical fluid cartridge ofclaim 13, wherein a flow area of the third fluid passageway is greaterthan a flow area of the second fluid passageway.
 15. The medical fluidcartridge of claim 14, wherein the flow area of the third fluidpassageway is at least 5 times greater than the flow area of the secondfluid passageway.
 16. The medical fluid cartridge of claim 13, whereinthe first, second, and third fluid passageways are configured so thatwhen fluid enters the fluid inlet port, a first portion of the fluidpasses through the second fluid passageway to the first fluid passagewayand then to the fluid outlet port, and a second portion of the fluidpasses through the third fluid passageway to the fluid outlet port. 17.The medical fluid cartridge of claim 16, wherein at least one of thefirst and second fluid passageways is a meandering fluid passageway. 18.The medical fluid cartridge of claim 16, wherein the body comprises awall that divides the second fluid passageway from the third fluidpassageway near the fluid inlet port and a wall that divides the firstfluid passageway from the third fluid passageway near the fluid outletport.
 19. The medical fluid cartridge of claim 1, wherein the medicalfluid cartridge comprises indicia that indicates a volume of the firstfluid passageway.
 20. The medical fluid cartridge of claim 1, whereinthe medical fluid cartridge is a dialysis fluid cartridge.
 21. Themedical fluid cartridge of claim 1, wherein the gap between the firstportion of the body and the second portion of the body allows the firstportion of the body to be disposed in a radio frequency coil while thesecond portion of the body remains outside the radio frequency coil. 22.A medical fluid system comprising: a medical fluid pumping machine; amagnet assembly defining a cavity in which a radio frequency device ispositioned, the magnet assembly being configured to generate a magneticfield within the cavity; and a medical fluid cartridge comprising: abody having a first portion defining a first fluid passageway and asecond portion defining a second fluid passageway in fluid communicationwith the first fluid passageway, wherein the first portion of the bodyis configured to be disposed within the radio frequency devicepositioned within the cavity of the magnet assembly, and the secondportion of the body is configured to be disposed within the cavitydefined by the magnet assembly and to remain outside the radio frequencydevice.
 23. The medical fluid system of claim 22, wherein the magnetassembly is part of the medical fluid pumping machine.
 24. The medicalfluid system of claim 23, wherein the magnet assembly is part of amodule that is releasably attached to a housing of the medical fluidpumping machine.
 25. The medical fluid system of claim 22, furthercomprising a medical fluid line connected to the cartridge to carrymedical fluid to the cartridge.
 26. The medical fluid system of claim25, wherein the medical fluid pumping machine comprises a pump that isoperably connected to the medical fluid line to pump medical fluid tothe cartridge.
 27. The medical fluid system of claim 22, wherein themedical fluid pumping machine is a dialysis machine.
 28. The medicalfluid system of claim 22, wherein the medical fluid cartridge isremovable from the cavity of the magnet assembly.
 29. A medical fluidcartridge comprising: a body comprising a first portion defining ameandering fluid passageway extending between a fluid inlet port and afluid outlet port and a second portion defining a fluid passagewayextending between the fluid inlet port and the fluid outlet port,wherein the meandering fluid passageway and the fluid passageway definedby the second portion are configured so that when fluid flows into themedical fluid cartridge via the fluid inlet port, a flow rate of fluidflowing through the meandering fluid passageway is less than a flow rateof fluid flowing through the fluid passageway defined by the secondportion.
 30. The medical fluid cartridge of claim 29, wherein themeandering fluid passageway and the fluid passageway defined by thesecond portion are configured so that when fluid flows into the medicalfluid cartridge via the fluid inlet port, the flow rate of fluid flowingthrough the meandering fluid passageway is 10 percent to 30 percent ofthe flow rate of fluid flowing through the fluid passageway defined bythe second portion.
 31. The medical fluid cartridge of claim 29, whereina flow area of the fluid passageway defined by the second portion is 5to 15 times greater than a flow area of the meandering fluid passageway.32. The medical fluid cartridge of claim 29, wherein the body isconfigured such that 10 percent to 30 percent by volume of the medicalfluid flowing through the fluid inlet port passes through the meanderingfluid passageway.
 33. The medical fluid cartridge of claim 29, whereinthe fluid passageway defined by the second portion is a substantiallystraight fluid passageway.
 34. The medical fluid cartridge of claim 33,wherein the body comprises a wall that divides the meandering fluidpassageway from the substantially straight fluid passageway near thefluid inlet port and near the fluid outlet port.
 35. The medical fluidcartridge of claim 29, wherein the meandering fluid passageway has afirst region and a second region, a flow area of the first region beinggreater than a flow area of the second region.
 36. The medical fluidcartridge of claim 35, wherein the second region of the meandering fluidpassageway is configured to carry fluid to the first region of themeandering fluid passageway.
 37. The medical fluid cartridge of claim29, wherein the medical fluid cartridge comprises indicia that indicatesa volume of a first region of the meandering fluid passageway.
 38. Themedical fluid cartridge of claim 29, wherein the medical fluid cartridgeis a dialysis fluid cartridge.
 39. A medical fluid system comprising: amedical fluid pumping machine; a magnet assembly defining a cavity, themagnet assembly being configured to generate a magnetic field within thecavity; and a medical fluid cartridge comprising a body having a firstportion defining a meandering fluid passageway extending between a fluidinlet port and a fluid outlet port and a second portion defining a fluidpassageway extending between the fluid inlet port and the fluid outletport, wherein the first portion of the body is configured to be disposedwithin the cavity of the magnet assembly, and the meandering fluidpassageway and the fluid passageway defined by the second portion areconfigured so that when fluid flows into the medical fluid cartridge viathe fluid inlet port, a flow rate of fluid flowing through themeandering fluid passageway is less than a flow rate of fluid flowingthrough the fluid passageway defined by the second portion.
 40. Themedical fluid system of claim 39, further comprising a radio frequencydevice disposed in the cavity of the magnet assembly, the first portionof the body of the medical fluid cartridge being configured to bedisposed in the radio frequency device.
 41. The medical fluid system ofclaim 40, wherein a second portion of the body of the medical fluidcartridge is configured to be disposed within the cavity defined by themagnet assembly and to remain outside the radio frequency device whenthe first portion of the body of the medical fluid cartridge is disposedin the radio frequency device.
 42. The medical fluid system of claim 39,wherein the magnet assembly is part of the medical fluid pumpingmachine.
 43. The medical fluid system of claim 42, wherein the magnetassembly is part of a module that is releasably attached to a housing ofthe medical fluid pumping machine.
 44. The medical fluid system of claim39, further comprising a medical fluid line connected to the cartridgeto carry medical fluid to the cartridge.
 45. The medical fluid system ofclaim 44, wherein the medical fluid pumping machine comprises a pumpthat is operably connected to the medical fluid line to pump medicalfluid to the cartridge.
 46. The medical fluid system of claim 39,wherein the medical fluid pumping machine is a dialysis machine.
 47. Themedical fluid system of claim 39, wherein the medical fluid cartridge isremovable from the cavity of the magnet assembly.